Over the last few years, there has been an increasing focus on the differences between the proper prayers of the Ordinary and Extraordinary Forms of the Mass.  More and more people are becoming aware that the spiritual nourishment and formation Catholics receive from the Ordinary Form is quite different in content and character to the Extraordinary Form. Whether one considers this a good or bad thing will depend largely on one’s general opinion of the post-conciliar liturgical reforms. However, if some form of the “mutual enrichment” desired by the Pope Emeritus is to emerge over the coming decades, it will have to involve much more in the way of detailed examination of the prayers of both missals.The present article is a small contribution to this. Since, at the time of this post, we are in Ascensiontide (well, at least we are in one of the liturgical calendars!), I thought it would be a good opportunity to examine some of the prayers for the Feast of the Ascension itself, namely the postcommunions.
In the 1962 Missale Romanum, the postcommunion prayer for Ascension is as follows (with my rough English translation):
Praesta nobis, quaesumus,This is a prayer that appears in this form in fifty-nine manuscripts,  ranging from the 8th to 17th centuries.  In nearly all of them, it occurs on the day of the Ascension itself. However, even in the few manuscripts that utilise this prayer on other days, the association of this prayer with Ascension is retained. In one 9th century manuscript it occurs on the Vigil, in one 14th century manuscript it is used in the Octave, and in five manuscripts (four from the 9th-11th centuries and one from the 16th century) it appears on the Sunday within the Octave. There is one other occurrence of this prayer in a slightly different form from the 6th/7th century, in the Leonine Sacramentary (also known as the Sacramentarium Veronense), again used on Ascension. 
omnipotens et misericors Deus:
ut, quae visibilibus mysteriis sumenda percepimus,
invisibili consequamur effectu.
Grant us, we pray,
almighty and merciful God:
that, what we have taken and obtained in visible mysteries,
may profit us by its invisible effects.
The collect, secret and postcommunion for Ascension Thursday were all replaced in the post-conciliar reforms.  With regard to the postcommunion, Abbot Patrick Regan, O.S.B., writes that the 1962 prayer is “well phrased but without connection to the feast”, hence why it was replaced, and he sees a “theological enrichment as we move from the Mass in the 1962 Missal to its successor in the 1970 Missal of Paul VI”.  The Ordinary Form’s postcommunion for Ascension reads as follows:
Omnipotens sempiterne Deus,This prayer is actually a combination of two different prayers,  both from the Leonine Sacramentary (omissions in red):
qui in terra constitutos divina tractare concedis,
ut illuc tendat christianae devotionis affectus,
quo tecum est nostra substantia. (2008 MR)
Almighty ever-living God,
who allow those on earth to celebrate divine mysteries,
grant, we pray,
that Christian hope may draw us onward
to where our nature is united with you. (2011 ICEL)
(1) Omnipotens sempiterne deus,
qui in terra substantia constitutos divina tractare concedis,
deprecantibus sanctis tuis,
ut eadem consequamur conversatione caelesti. (Leonine 689)
Almighty ever-living God,
who allow in earthly realities the celebration of divine mysteries,
by the intercession of your saints,
that we may obtain the same heavenly way of life.
(2) Tribue, quaesumus, domine,This is an example of a technique called centonization, used with some regularity by the reformers. Centonization takes parts from two or more different prayers and weaves them together, sometimes along with originally-composed parts, in order to make a new prayer. It is not a modern technique; Pope St Gregory the Great used it when he composed the collect now used on the 1st Sunday of Advent in the 1962 Missal.  The character of the centonization used by the post-conciliar reformers is often rather different to that of Pope St Gregory, as we will see.
ut illuc tendat christianae nostrae devotionis affectus,
quo tecum est nostra substantia. (Leonine 185)
Grant, we pray, O Lord,
that our Christian hope may draw us onward
to where our nature is united with you.
The first prayer of the above two used to compose the Ordinary Form’s postcommunion, Omnipotens, occurs only in the Leonine Sacramentary; there are no other occurrences in the tradition.  The second prayer, Tribue, has quite a wide use across the 6th to 15th centuries in slightly differing forms, with a roughly 50/50 split between use on Ascension and as an Easter prayer for St Andrew.  In the form above, however, it occurs only in the Leonine Sacramentary, and is associated with the Ascension. Neither of these prayers is used anywhere in the 1962 MR.
It looks like that the Consilium were very keen on using the postcommunion prayer for Ascension from the Leonine Sacramentary, rather than retaining the one from the 1962 Missal. In all likelihood, this is because the Leonine prayer, like the Preface for Ascension,  speaks about our natures being united with God. However, since there is no mention of the faithful having just received the Eucharist (something the reformers dogmatically insisted that a postcommunion must have), the prayer had to be centonized with the first half of another prayer that the reformers also liked the phrasing of, which in this case happened to be from the same sacramentary. This particular centonization, however, is a little different to the example of Pope St Gregory the Great given above. Instead of centonizing two thematically similar prayers from the same time of year, one of the prayers the reformers chose, Omnipotens, was only ever used in the month of August, on the feast of St Stephen buried in the Catacomb of Callixtus on the Via Appia, and has never been associated with Ascension. And the other prayer, Tribue, as we have already mentioned, is only associated with Ascension in about half of the manuscripts that contain it.
We have thus moved from a postcommunion prayer in the EF that has always been associated with the Ascension, and almost universally on the day of the feast, to a newly-centonized composition in the OF made up of one prayer that never had anything to do with the Ascension and another only used as such around half the time, and that mainly on the Vigil rather than the day itself. Omnipotens and Tribue do at least have similar themes. Omnipotens, however, is only extant in one single manuscript, the Leonine — admittedly an important, early witness, but it is perhaps questionable whether it ought to have been used to replace a far more widely and consistently used prayer, the source of which, incidentally, is also the Leonine sacramentary!
If we look for a moment at the collect and super oblata as well as the postcommunion, we can see that, as a result of the Consilium’s changes, the prayers of the OF Ascension are, on the surface, more thematically focused than those of the EF. The prayers, along with the preface, all make explicit mention of the ultimate end of the Ascension: the union of our natures with God in heaven, our divinisation. “[W]here the Head has gone before in glory, the Body is called to follow in hope” (collect), “that through this most holy exchange we, too, may rise up to the heavenly realms” (super oblata), “that Christian hope may draw us onward to where our nature is united with you” (postcommunion).
One can understand how this might be seen as an enrichment in terms of theology. However, this has the effect that the OF prayers are arguably more limited in scope than those of the EF. Since the focus of the OF prayers is almost exclusively on the eschatological significance of the Ascension, there is very little space left for consideration of what the fruits of this feast ought to be in our daily lives. In the postcommunion, for instance, there is no mention made “of the fruits of the mystery just celebrated” (cf. GIRM 89); the closest we get is that we have been allowed to celebrate it. We pray that “Christian hope”, rather than “these mysteries”, may draw us onward to heaven. In the EF, on the other hand, space is given to both the here and now as well as the end of time: “that, what we have taken and obtained in visible mysteries, may profit us by its invisible effects” (postcommunion), “grant that we may be delivered from present dangers and attain eternal life” (secret), “that he might make us sharers in his divinity” (preface), “that we… may in spirit dwell already in heavenly realms” (collect). The scope of the EF prayers is wider, more complete than that of the corresponding OF prayers. 
The visible/invisible parallel in the 1962 postcommunion also has resonances with the Ascension. Our Lord visibly ascended (Acts 1:9), and has given us the Holy Spirit to be with us invisibly (Jn. 14:16-17; 16:7-11) until He comes again visibly in glory (Acts 1:11; Mt. 24:30; Rev. 1:7). Jesus Christ, physically and visibly present with His disciples until His Ascension, and now invisible to us until He comes again, can be seen today at every celebration of Mass under the visible signs of bread and wine in the Most Blessed Sacrament. It is also a literarily pleasing parallel: outside of the Creed, it is actually quite uncommon in liturgical prayers, and contributes to the conceptual variety of the EF prayers for Ascension.
Whether one set of Ascension prayers is preferable or superior to the other, and whether the different liturgical sources have been used in a manner consistent with the organic development of the liturgy, is a matter for reasoned debate. But in order to begin to appreciate how the OF and EF each shape the faith of Catholics, and the similarities and differences in how the two Forms of the Roman Rite accomplish this shaping, it is absolutely vital to begin the detailed work of delving into the sources of the prayers of each Missal.
 The main example is Lauren Pristas, The Collects of the Roman Missal: A Comparative Study of the Sundays in Proper Seasons Before and After the Second Vatican Council (London: Bloomsbury, 2013).
 This and other similar data is taken from E. Moeller, J.-M. Clément & B.C. ’t Wallant (eds.), Corpus Orationum (Turnholt: Brepols, 1992-2004), a 14 volume set of books that detail the history and sources of almost every single extant Latin prayer, excluding prefaces (henceforth referred to as CO). It is a vital resource for detailed research into individual prayers of the Latin liturgical tradition.
 Two manuscripts from the 8th century, eleven from the 9th, eight from the 10th, fifteen from the 11th, six from the 12th, and twelve from the 13th century or later. Cf. CO 4387b.
 Praesta nobis, omnipotens et misericors deus, ut, quae visibilibus mysteriis celebrando suscepimus, invisibili consequamur effectu: Leonine 172; cf. CO 4387a.
 However, the Ascension Thursday collect of the 1962 Missal was inserted into the editio typica tertia (2002) of the OF Missal as an alternative collect.
 Advent to Pentecost: Comparing the Seasons in the Ordinary and Extraordinary Forms of the Roman Rite (Collegeville, MN: Liturgical Press, 2012), p. 282.
 Cf. A. Dumas, O.S.B., “Les sources du nouveau missel romain (II)”, Notitiae 7 (1971) 74-77, p. 75
 Cf. Gelasianum Vetus, 1120 (also 1141 and 1149, which have the same introduction) and 1158; for more detail, see Pristas, Collects of the Roman Missal, pp. 32-38.
 Cf. CO 3972.
 Cf. CO 5924. The other usages are as follows: one variation in the Gelasianum Vetus (8th century), used as an Easter evening prayer; another variation in twenty-six manuscripts, from the 8th-12th centuries, all but one used on the Vigil of Ascension as a postcommunion; a third variation found in thirty-four manuscripts, from the 8th-13th centuries, used as an oratio (collect) for St Andrew.
 Note that the Preface of the Ascension in the 1962 Missal was retained as Preface II of the Ascension in the 1970 Missal. Incidentally, this is one of the very few examples where a preface from the preceding liturgical tradition has been carried over into the post-conciliar Missal with no changes to the text.
 One of the criticisms often levelled at the Ordinary Form is that it lends itself too easily to didacticism. This criticism can also be applied here: the reformers seem to have insisted on all the proper prayers for Ascension Thursday explicitly mentioning the Ascension itself, which could perhaps be seen as overly didactic (and also rather heavy-handed).