Is piss off used in American English? How does it differ from the BrE usage?
I can't speak for American usage. But in British usage, people say "To piss somebody off" = "to annoy them".
In the passive, "pissed off" means angry. [I have heard Americans use "pissed" in this way. This is now recognised here because of American films and TV, but when I was younger, that only meant "drunk".]
"Piss off" as a intransitive phrase (usually as a vulgar command) is quite different - it may indicate that the speaker is angry, but it doesn't mean "annoy somebody".