The compression fitting on the oil line is a "one time use" item. Since it has slipped away from its original position, it will leak if the line is reinstalled. Cut the line back a little and acquire a new compression nut to install the line. The first time a new nut is tightened it swages itself into the copper tube, creating the indented section in the tubing and sealing itself. NAPA stores used to sell that nut- - - - -but they're getting a little scarce since they haven't been used much in the past 50+ years. There is a corresponding female fitting that the nut threads into.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!