Why are you removing all of the oil base paint? Concentrate on the loose areas. If the paint is fully adhered to the wall there isn't a need to remove it. Any remaining paint can be primed over.
Large loose areas can be removed with a stiff 6 inch taping knife. You might have to use a stiff putty knife to get things going.
Damaged areas can be repaired after priming with a good oil base primer, like Kilz.