Green’s Dictionary of Slang

osher-kah adj.

[backsl. kosher adj. (1)]

(Aus.) satisfactory, as required; legitimate.

[Aus]R.G. Barrett Mud Crab Boogie (2013) [ebook] ‘Yeah? It’s all … you know. Osher-kah, Neville old mate?’ ‘Of course. Why? What makes you say that?’.