I don't think Oni is a bad character. They did improve his toward mk which was already pretty good. It's now on par with Ken's step kick in terms of startup. Losing invincibility on his ground pound was a pretty big nerf but he does have DP FADC now. I think Oni player's will have to change their styles to be a bit more conservative.
step kick was pretty worthless due to the startup, plus the range it actually covered, I think Ken's stepkick goes farther than Onis. Of course I could be wrong, but I still don't think his step kick will be used that much more.He got FADC on his dp, but they nerfed his dp overall, making it less invincible and making him unable to combo anything after a dp>FADC on hit. Add to that his mk demon slash nerf, his health nerf, and his stomp nerf and Oni just got worse. Oni players will have to play more conservative because they nerfed a lot of his options making him even more unsafe when trying to mix people up. Buffs like reduced frame disadvantage on mk slash and stomp don't mean anything considering the above nerfs and the fact that they're still pretty negative on block(I think -4 and -6 respectively) so it will still be very easy to punish regardless.
I was training with oni for him to be my projectile character to offset some bad matchups i'd have playing yang/elena, but I'll probably join the e.ryu train as well.