Green’s Dictionary of Slang

1. (Aus.) to go away; also as imper. [piss off v. (1)].

1918

Aussie (France) Aug. 12/1: Am awakened, given an answer, and told to p.o. at the ‘toot’ (p.o. means ‘push off,’ I think). I p.o.

2. (US) to annoy [piss off v. (2)].

1967–8		Baker et al. CUSS 167: P.O. / Piss off / Tease or annoy someone.
2002		‘Touré’ Portable Promised Land (ms.) 265: What was the thing they wrote on the wall of his little sister’s school that P-O’d him?