Old Burmese is the language attested in the stone inscriptions of Pagan

Bagan

Bagan , formerly Pagan, is an ancient city in the Mandalay Region of Burma. Formally titled Arimaddanapura or Arimaddana and also known as Tambadipa or Tassadessa , it was the capital of several ancient kingdoms in Burma...

 and is the oldest phase of Burmese linguistic history. The earliest monument in Old Burmese is the Old Burmese face of the Myazedi inscription

Myazedi inscription

Myazedi inscription , inscribed in 1113, is the oldest surviving stone inscription of the Burmese. "Myazedi" means "jade stupa" , and the name of the inscription comes from a pagoda located nearby...

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Many things remain controversial in the understanding of Old Burmese phonology and morphology. However it is clear that Burmese maintains a number of distinctions which are lost in the standard orthography of standard Burmese. Examples of such differences include the consonant yh- and the lateral clusters kl- and khl-. The earliest Old Burmese documents, in particular the Myazedi and Lokatheikpan

Lokatheikpan

The Lokatheikpan is a temple in Pagan Burma, founded circa 1125. This temple is known for its frescos, which contain painted inscriptions that are among the oldest documents in the Old Burmese language.-References:...

inscriptions frequently have -o- where later Burmese has -wa. Old Burmese also had a final -at and -an distinct from -ac and -any as shown by Nishi (1974).

Old Burmese	Written Burmese
yh-	rh-
pl-	pr-
kl-	ky-
-uy	-we
-iy	-e
-o (early OB)	-wa