Copper water water line is out of round slightly
I do not think that teflon tape will help with an out of round tube in a compression fitting. My first thought would be to try to restore the tube to roundness by proper use of Vise-Grips with a rag to prevent scoring of the sealing ring. There is a die clamp device designed for gripping tubing to make a flare in the end. This die might be used to restore the tubing to roundness. You might have to remove the ring seal and put on a new one after truing the end of the tubing.

But be prepared to cut off the out of round end and join a fresh piece of good tubing.