I think there are a few misconceptions here--I'm pretty sure I remember reading of the various hunting numbers--how many deer, elk, sheep, etc--that each had killed, and O'Connor had killed wayyyyyyy more game than Keith.
As far as their ideas, O'Connor of course was known for his advocacy of the 270, and the first rifle I bought when I got my first degree and had enough money to not just buy the cheapest thing I could find was a Winchester Featherweight in 270. It took my largest deer to date, and it's both a classic caliber and rifle IMO. One thing to keep in mind--O'Connor was notoriously fond of sheep hunting, and as such a cartridge with some reach was going to be a favorite.
One of my other first rifles was a marlin 35 Remington, though, so I guess you could say I don't mind big, slow bullets either.