Typically OM lenses have set screws under the rubber grip. Carefully remove the grip and don't stretch it or you will be very sad.

The set screws can be tightened once you set the ring correctly and then a drop of nail polish or lacquer will keep them from moving. Some new adhesive will retain the grip. I have used many rubber bands to hold a grip tight while the adhesive sets.

Such a nice lens might deserve a trip to our favorite OM expert or other pro.