I guess I have always been loathe to point to a performance spike SOLELY and say, "Hey! PEDs!"
In fact, in O'Neill's case, he did NOT start hitting 40 HRs a year in NY, but rather was allowed to finally be the hitter he always wanted to be (perhaps the hitter that his body needed to be). For a big guy, he was a line-drive hitter who liked to use all fields. That's what he did in NY, to that organization's credit, they allowed him to be who he was as a player. His top HR year was in 1991 with 28 (that was with the Reds). In NY, he settled into being an 18-25 HR a year guy. Who was finally allowed to hit for average and use all fields. Probably why his doubles spiked and one of the drivers for his higher OPS'. As did his taking far more walks.
I always wish that had happened in Cincy...
O'Neill's career stats page from baseballreference.com: