as others say, the new o-ring might have been pinched and torn. was it
tight going back and did you use the distributor bolts to get it back
into its socket?
check again, and check the mount area. on split facings, you can get
oil seepage along the groove obviously.
if the oil is found to be leaking from the distributor's internal shaft
seal, you'll need to replace that. most distributors are not built to
be consumer repairable so if it's leaking, you're probably best advised
to get a fully reconditioned distributor.
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nomina rutrum rutrum