How do I get trained in Oracle?

Post-Crisis Supergirl does not arrive on Earth until Barbara has already established herself as Oracle. Many of her earlier adventures (including some shared with Batgirl) were retroactively participated in by Power Girl instead. In addition, Barbara and Dick Grayson had been lovers at some time in the past, and she had once been engaged to Jason Bard, who is no longer a Vietnam veteran but is still a private investigator (and former Gotham City policeman).