With OWB's, internal or external, I take a piece of good paper towel (not one that falls apart when wet), twist it to make it a tight twisted piece that will fit inside the OWB. Then I soak the towel with DA and feed it into the bearing. Then I spin the bearing by hand in both directions for a while on the towel to work out any built up oil/gunk in the bearing.
Then a drop or two of after run oil and reinstall.
Usually though, once they start slipping, it's a downhill spiral until you replace it and the starter shaft with new ones. Don't put a new bearing on an old starter shaft as both the bearing and shaft tend to wear. It's a good idea to replace both at the same time if that's what you end up doing.