glenn totally ditch that o ring, and get some loctite silicone gasket sealant and put a bead of this around the "L" edge of the diff case, then close it up, clean up the excess that squeezes out once the screws are put in with a cotton ear bud.

put the sealant on the edge, and be sure not to go too close to the inside part of the diffcase, avoid it going into the diff when you close it up with the screws

after this, it will never leak, maybe only a fraction on the outdrives but will stay good for at least a full year of racing easy