Interesting question -- I believe that the answer is that Homer, Ovid and Virgil all wrote in (essentially) the same meter (hexameter, mostly dactylic). They did use some of the same characters (namely the Olympian gods, and, in the case of Virgil and Homer, some shared human characters from the story of the Trojan War). Ovid and Homer certainly wrote down mostly oral tales (Ovid, in the Metamorphoses, compiled many of the Greek and Roman myths, many of which were oral tales of many hundreds of years standing), but Virgil only took a part of his story from oral tradition -- a large portion of it he invented (the particulars of much of the Aeneas story). So you could say that all three authors did all three things, but the one clear unifying characteristic that all three authors use is the characters of the Olympian gods.
Homer, of course, wrote in an early form of Classical Greek. By Ovid's and Virgil's day people certainly could still read this text (in fact, educated Romans of that time, such as Ovid and Virgil, all were well trained in reading Greek), but it was a literary language that was significantly different from the Greek spoken at that time. Ovid and Virgil, of course wrote (and spoke) in Late Classical Latin. Nevertheless, all three poets wrote in hexameter (the six-foot line), even though there are signficant differences between the languages, and certain poetic conventions must be changed to accomodate them. Homer had established the hexameter as the epic poetic form, so both Ovid and Virgil (and other poets of the time) were, in essence, imitating Homer.
Homer's story, of course, is the oldest of the three, and, possibly, the least adulterated. This is difficult to prove, however, since there is very little literature in Greek before Homer (mostly fragments or accounting records). The religious system of Ovid and Virgil's Rome was inherited, at least partially, from the original Greek pantheon of gods. Many of the tales of Roman mythology are re-tellings or re-workings of earlier Greek tales, changed to fit local Roman lore. So while Ovid, Virgil, and Homer all drew from the same well of shared pagan mythology, Homer predated Ovid and Virgil by (at the very least) six hundred years, and therefore had a different perspective and (possibly) access to older tales than Ovid and Virgil. It is without a doubt that Ovid's, especially, tales are, in some cases, of very great antiquity. But Ovid's tales are a collection; Homer's work is more of a "snapshot" view of a Mycenaen tale of his time, while Virgil's is another compilation injected with a large dose of poetic invention (and imperial flattery!) These three works of literature are each very different from each other, so it is difficult to say that they each "came from oral tradition". Probably the purest example of that came from Homer, followed by Ovid (some of his tales had, of course, been written down long before he collected them), and lastly Virgil's Aeneid.