Is the OT unveiled in the entire NT or just the Gospels? St. Augustine Sayeth:


  the New Testament lies hidden in the Old and the Old Testament is unveiled in the New.


(paraphrased quote from the Catechism)

Just for context, I looked this up and he actually said


  Wherefore, in the Old Testament there is a veiling of the New, and in the New Testament there is a revealing of the Old.


source

We believe this in Catholicism and it's apparent from the way we structure the readings at Mass, the OT reading is always subtly explaining the Gospel one.  But is that the case, is it just the Gospel (+Acts?) that is hidden in the OT.  How do the epistles and revelation work in to the "hidden in / unveiled" equation?
I think the whole NT reveals the old, particularly in explaining OT prophecy. Possible the best example is the Book of Hebrews, since it may well have been written for exactly that purpose. Some examples:

To which of the angels did God ever say,
&quot;Sit at my right hand
until I make your enemies
a footstool for your feet&quot;

Hebrews 1:13, quoting Psalm 110:1.
Then there is the account of Melchizedek in Hebrews 7, who as both King and Priest was an archetype of Christ. (This theme is carried on into subsequent chapters, the High Priest in chapter 8 and the Blood of a sacrifice (Christ in NT) in chapters 9 and 10.)
Finally (in this very brief overview of Hebrews), is the men and women of faith in chapter 11.
That's just one book!
Other examples I could mention are the imaginary of Ezekiel and Daniel coming back in Revelation, Paul explaining grace in Galatians 4:21-31 using the examples of Hagar and Sarah and Peter quoting proverbs in 2 Peter 2:22.
Finally (for my summary, there is of course much more), there is the promise of a New Heaven and a New Earth, first revealed in Isaiah 65:17 and 66:22 and then repeated by Peter in 2 Peter 3:13 and John in Revelation 21:1.
So, yes, the OT comes back time and time again in the NT. One might speculate it was planned that way. :)