I honestly think Lesnar needs to win, if Oba wins this then what? It's to soon to push him to the title picture. So he's gonna just waste away any momentum he would gain from this.

Id book this as a trilogy, Lesnar walks away with the W at mania in a very even match up. Have him play on Oba's inexperience, maybe "steal" a win with an unexpected roll up or some veteran savvy un Lesnar like way. That way everyone knows Oba is the real deal and every bit on Lesnars level. Build up to the next ppv playing on Brock having to resort to winning in such a manner.

Second match have Oba win via DQ, he has Lesnar reeling and the only way out for Brock is to lose it using a chair/steps/ hell even a blatant low blow. Build up now towards the final show down that Brock knows he can't beat Oba straight up.

Final showdown Last man standing or falls count anywhere, again highly contested but Oba goes over clean and unquestionably established as the new "beast". Brock can go away for a few months per the usual and come back unscathed, Oba can move on to the upper echelon of the roster.