im assuming oac and oap refer to one arm chin and one arm pull? i'll refer to them as the same thing for convenience though.
with reference to other sites such as beast skills etc, i believe doing negatives helped me to improve. i wish i could have trained with weighted pullups, but lacked equipment.
what i did was to use towel pullups. a towel on the bar, one hand grabs the low part of the towel, other hand normal on the bar. 3x5 for each side. put as much weight as you can on the non towel hand. after all, the only one you are cheating is yourself.
after a while, a few months or so, try doing controlled negatives. most probably you will still fall too quickly. if so, have the other hand hold onto the towel as you lower yourself.
an important note is not to overtrain. the oap seems to very strenous on the shoulder joint, and anecdotal evidence seems to show injuries for people who rushed into doing oap with lousy form too quickly. take it slow.