95 altima transmisson problems.

i have a 95 nissan altima gxe and the transmission is slipping. it shifts late into second gear, but after that seems to run pretty much ok with little slippage. there is also a dent on the bottom of the tranny fluid pan. i was wondering if anybody knew if i removed the dent if that would help the transmission run better? i heard its possible but i'm not sure. i also have a O2 sensor out and a knock sensor that is on that caused the "check engine" light to come on. i have also heard that by the O2 sensor being bad, that could cause the transmission to slip. is this true? would replacing the O2 sensor help the tranny? if anybody could help answer these questions that would be great. i would like to avoid having to have the tranny replaced if all possible... if i can't does any body know where i can get a new or used transmission for cheap? i can do the labor. thanks