According to http://en.wikipedia.org/wiki/Timeline_of_Occupy_Oakland it was the use of tear gas and beanbag rounds on Oct 25. This resulted in Scott Olsen (ex-marine and Iraq veteran) suffering a skull fracture and needing to be hospitalized.

That timeline gives Nov 2 as the first truly violent act by protestors, namely the setting of a police barricade on fire while the police were attacking the protestors with tear gas, etc. (I'm not counting the tearing up of deposit slips on Oct 22 as violent - and the protestors cleaned up the mess that they caused on that occasion.)