The interrupt part? I'd say maybe control.
It's possible that combo helps with the repulse part.

The "Failure" part of his standard has a set %.
So to my knowledge, nothing.

no i mean his basic attack has a chance of failing at all and not doing anything, maybe there is a stat that can make him fail less

Probably combo, that's what decides which attack gets picked for others characters. For example Zabuza has 3 attacks, 1 hit atack, 2 hit atack with a more damage, 3 hit atack with most damage and repulse, and with high combo he will do 3 hit more often and 1 hit one less often. So i am guessing it's same for Onoki, with high combo he will fail less and do repulse one more.

I do not believe that there's any stat to prevent that.
But to offset it, Onoki does have the boulder jutsu passive which gives 1 std attack to a random ninja.
(of course if it lands on Onoki and both times he fails... then may the odds ever be in your favour.)

Same as Durs, I believe that there isn't any stats to prevent it. I will try and see if I can get a response from the team.

If he fails both times then the odds WEREN'T in your favor.

Plus if he fails then his skill that gives ninjas on your team an extra standard attack becomes useless. Had you taken a ninja without that skill but who didn't fail, then they would have an extra skill. So maybe they'd have a summon and an attack as opposed to a failure and an attack.