Ostler started out as a boy player in the Children of the Chapel troupe; he was cast in their 1601 production of Ben Jonson's The Poetaster, with Nathan Field and John Underwood, two other future King's Men.
He was praised for the quality of his acting, once being called "the Roscius of these times" (John Davies, Scourge of Folly, 1610).
Ostler also became a shareholder, or "householder" (i.e. a part-owner) in both of the King's Men's theatres, the Blackfriars (20 May 1611) and the Globe (20 February 1612).
By common law, his property should have passed to his wife; but John Heminges seized control of his deceased son-in-law's Globe and Blackfriars shares.
Thomasine sued her father to recover her property; the outcome of the suit cannot be determined with certainty from the surviving records, but John Heminges appears to have been able to retain control of the shares.