Their monsters met during the wolf's first turning. Interpretations differ as to who flinched first, but they both backed away.
They barely spoke to each other in Sunnydale. Then again, Angel and Oz barely spoke, period, in Sunnydale. Several months after graduation, though, they met up again in LA when Oz delivered the Gem of Amarra to Angel and helped rescue him from a poker-wielding Spike and a truly creepy pedophile vampire.
More importantly, though, Oz saw Angel in the sun.
They kept in touch.
After leaving Sunnydale the second time, Oz headed back to Tibet. He'd told Willow he shouldn't be around people until the wolf was under control. Angel, though, isn't exactly a person.
Oz turned back.
Kita writes Angel. Gloss writes Oz.
Monsters Ball: The main drafting center.
Supporting stories linked below.
November, 1999 - April, 2000: They correspond.
May, 2000: Oz turns back for LA. It takes him awhile to find Angel, since Wolfram and Hart bombed the offices and Angel moved into Cordelia's apartment in Silverlake.
June, 2000 - August, 2000: Angel puts Oz in a safe house. Monstrosity ensues, including Oz's liaison with Lindsey.
September, 2000: Angel becomes a champion. Oz goes back to Tibet.
May, 2001: Buffy dies. Angel goes to Tibet to mourn.
August, 2001: Angel stops by Oz's hut. Monstrosity and humanity ensue.
September, 2001: Oz accompanies Angel back to LA and moves into the hotel.