Also, the previous fitting had a female coupling on the bottom and the
copper pipes seem to be attatched to what I presumed was the old copper
o-rings! Unless I can get these off, the old fitting nut wont come off,
and I cannot fit the new parts anyway.

Often you can use the old olive and nut on the new fitting.

If not, removing the old olive can be difficult. You can get special olive
removers, or you can hacksaw a slot on the olive and try to ping it off with
a flat blade screwdriver in the slot. However, if you nick the pipe itself
with the hacksaw, it might never seal, so you must be careful.

Christian.