Almost impossible - you could try filing the ring with a needle file, but why do you need to do this? If you want to extend the pipe, you can use a new compression fitting with the old ring and nut. If it's damaged, you can cut about an inch of the end of the pipe, and replace it with a new bit, again with a compression fitting, or solder fitting if you're comfortable working with these.
It is possible to remove an olive from copper pipe using a hack saw and a flat bladed screw driver. Carefully cut the olive diagonally across, being careful not to cut into the copper tube underneath, then taking the screw driver insert the tip into the groove and twist. The olive will split along the cut, or it will with practice....lol
(Failing that Monument Tools make an olive remover, but it's an expensive toy for the diy'er)
If the joint has been overtightened then you will probably not be able to get it off. If it hasn't been overtightened then just undo the nut slightly, so the it's fairly loose, and use a spanner that's about 1 or 2 mm bigger than the pipe, place the spanner round the pipe with one side against the the nut and tap the spanner with a hammer to pus the joint off the pipe, with the olive inside it.
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