Well, I wanted to wait for few votes to role in, but I guess I'll get the ball rolling.
Kessler has a slight advantage, as he scales to a ~347.2 megaton feat that was performed by a Cole who was far weaker than when he and Kessler fought.
Ozpin scales to a ~337.2 feat, if we multiply Raven's storm feat by 4. And he's supposedly only been slightly weakened as, so its a close call.
Kessler has the more impressive power set: multiple summons to either attack or distract (though Ozpin's Aura will probably protect him from phantasm mindscrew, as they need to make physical contact in order to work), the ability to turn Oz's bioelectricity against him, mind hax via memory inject, a telekinesis that has a multi-city block range, has seemingly endless amounts of electricity to use, and regains stamina extremely quickly.
Ozpin might have an edge in experience, and his flight is worrisome, but Kessler resists heat (fire) and lightning fairly well, ice can be warded off through the extreme heat Kessler's lightning puts off (which was brought up in the comics), and telekinesis counters flight pretty hard (imagine Kessler pulling a Scorpion on flying Ozpin). The rest should be countered by Kessler's regen and passive barrier. Old Man Cole's insane endurance should also let him tough it out, man took an extended beating and a point blank lightning storm, and he still managed to get up and blitz Cole one last time to show him a vision.
Kessler 7/10