From Ashley Mallett's popular cricinfo piece..
Bradman, who reckoned O'Reilly to have been the best bowler he saw or played against, said he had the ability to bowl a legbreak of near medium pace that consistently pitched around leg stump and turned to nick the outside edge or the top of off.
Bradman once wrote me: "I always classified Clarrie Grimmett as the best of the genuine slow legspinners, (I exclude Bill O'Reilly because, as you say, he was not really a slow leggie) and what made him the best, in my opinion, was his accuracy. Arthur Mailey spun the ball more - so did Fleetwood-Smith and both of them bowled a better wrong'un but they also bowled many loose balls. I think Mailey's bosey was the hardest of all to pick.
Don't believe Bradman ever saw Barnes play but I assume that Barnes/O'Reilly would be similar bowlers, O'Reilly perhaps a little slower? Whilst Grimmett would be more of a classical leggie like Warne.
All I can say is I'd love to have seen either one bowl, it would have been quite a sight. No doubt both are in the top 5 spinners of all time.
Ashley Mallett on Bill O'Reilly and Clarrie Grimmett | Cricket | ESPN Cricinfo
Bradman v Warne
Clarrie Grimmett: The best ever spinner born in NZ on 25 December 1891 | The Roar
"Grimmett’s problem was that he often talked about the fact that he could get Bradman out. The Don never liked bowlers who made this sort of boast. Cricket historians believe that this mutual hostility was the reason why Bradman insisted on dropping Grimmett for the 1936/37 Ashes series in Australia and from the 1938 tour to England."