Just for fun, I went to the Southern League batting leaders pages at B-R. Wanted to see how 21 year olds who hit "like" O'Neill (north of .800) fared when they made it to the bigs, seeing significant playing time, at age 22. I only looked at 21 year olds...ignired some 20 year olds and one 19 year old.
I initially looked at 2005, picking it randomly because it wasn't too far back and might give us some decent data. Thien I went to 2006. Then I jumped to '12 and '13 just to make it more recent.
2005 had bunch of hitting 21'ers who played a decent amount in the bigs at 22. Here is their OPS in the bigs at 22.
Jeremey Heredia .700 (his AA game was built on BB's not blast)
James Loney .901 (111 PA's)
Jeff Francouer .782 ( hit .884 when called up as a 21 year old)
2006
Matt Kemp .894 at 22 in the bigs.
Tony Abreu .713
2012
Nick Franklin .686 at 22
2013
Javier Baez .551 at 22
Christian Yelich .764 at 22
Arismendy Alcantara .621 at 22
There are some failures in that (short) list...and some successes.
Maybe the most similar to O'Neill were Francouer, Kemp and Baez. They wre boppers of some sort who slugged .487, .528 and .535 in AA. Tank was at .508.
Francouer K'ed in 20% of his PA's. Kemp was 16%, Baez 27% and Tank 22%. Francouer had a .25 Eye, Kemp a .5, Baez a .3 and Tank a .4.
Kemp did go to AAA at 21 and then the bigs that year, too.
So, based on this not-worth-much study, O'Neill might bomb next year (were he to get an early call) and hit .600 or less, like Baez. Or he might bang out a .780-type number like Francouer did. Wouldn't that be nice? Or he goes all Kemp on the league and completely rips the ball.
2 out of those 3 guys did pretty well with an accelerated path to MLB.
To get to a 110 OPS+ (as I suggested he might), O'Neill would have to hit .765, or so. Say .260-.310-.455. That would make him have to look an awful lot like Guti did this year.
It isn't out of the realm of possibility. But I'll round his projection down to 105 just for caution's sake. I'm always too dang cautious.
But he would be just fine after a short stint in Tacoma. Maybe better than fine.