In the case of the Avesta the distinction between "oral" and "written" transmission is not as sharp as we could expect. All the main Avestan texts were composed before the Achaemenid time or in the early Achaemenid times. Although we don’t know the exact date of the invention of the Avestan script, the possible dates range from the 3th.C. A.D. to the the 8th.C. A.D. (the most probable dating is around the 5th C.). During this first period the Avestan texts were mostly transmitted orally, although we can’t exclude that occasionally some texts also were written. The traces of a written version of the Avesta or of some Avestan texts before the Islamic period are very scarce:
- 1. Mentions (mostly crearly anachronical) in the Zoroastrian tradition of a writtten version of the Avesta (e.g. DkM 459.22-460.8; XR 8-10). Legendary are the notices about a prealexandrinian Avesta.
- Mani (Kephalaia 71.2-33) mentions that Zaraθuštra did not write, but his disciples did it.
- 2. Plinius (Hist.Nat. 30.3-5) mentions a notice fro Hermippos, who seems to have seen a voluminum with two millions of Zoroastrian verses.
- 3. Pausanias (Graeciae descriptio 27.5-6) describes a fire ritual in Lydia and the priest rsings readng from a book in 2th. Century.
These rare occasions in which Avestan texts were written, if there were any, did not have a noticeable influence on the Avestan transmission.
After the invention of the Avesta script some of the Avestan texts were written, although it is not easy to determine which ones. Probably the texts recited in the ritual (nearly the only ones that have survived) were not written independently, but only within the big collection we know as the Great Avesta (Kellens JA 1998). In the ritual written texts were not used: the texts were learn by heart and transmitted mainly orally. The written texts were not thought for the daily usage. They were probably rare and contained most probably only the texts of the Great Avesta. But none of these manuscripts containing the Great Avesta has survived. Despite the invention of the Avestan script the main transmission's stream kept oral.
Most of the different types of the extant Avestan manuscripts probably go back to one single manuscript. In the case of the Vīdēvdād we know al least about two historical occasions where only one manuscript was available: before and after the addition of the Pahlavi translation to the Avestan text (H. Humbach  "Beobachtungen zu der Überlieferungsgeschichte des Awesta", MSS 31, pp. 109-122; more details in the descripton of the Vīdēvdād transmission). The fact that Pahlavi- and Sāde manuscripts share some faults and omissions speaks for a common source of both transmission lines. In the case of Yasna, all the Pahlavi manuscripts also go back to an unique one. For the copy of this manuscript the Avestan text was taken from a manuscript and the Pahlavi translation from another. This didn’t happen ca. 1110, as generally assumed, but probably eben before 1020 (Cantera-De Vann, "The colophon of the Avestan manuscripts Pt4 and Mf4", Studia Iranica 34 (2005), 31-42). Consequently, it is very likely that before the 10th Century the main stream of the transmission was oral, not written
In fact we don’t have copies of Avestan manuscripts till very late. The oldest available one is K7a,b. K7a contains a Pahlavi-Visperad, and K7b a Visperad-Sāde. Next come three manuscripts from the year 1323: K5 (Pahlavi-Yasna), J2 (Pahlavi-Yasna) and L4 (Pahlavi-Videvdad). K1 stems from 1324. All other manuscripts are quite younger. Neverthless, the colophons of the manuscripts mostly allow us to follow the history of the written Avestan transmission somewhat earlier, nearly till the 10th century, depending on the text and transmission line. Most of the extant manuscripts are from the 18th Century.