(Beware I did not spell check.)
You have not shown any pics what is left of the Oil Pressure line meaing where you cut it. So it is hard to come up with a repair.

What happens is rust builds up between the Oil Pressure Gauge Line and the Hex headed Fittig and bonds them together.

Another way to have done it would have been to remove the Plate that the Oil Filter Line screws into as seen in the Pic. but before I realized I should have done that I also cut the Line.

I cut the line as close a possible to the Fitting. Thinking that that after I got the fitting cleared and out I would simply shover the end back into the Plastic Tubing.
Unfortunatly the old Plastic is inelastic and I only got it part way back on and then stuck and would go no further. I think I got it on like 1/4 of an inch.

I am not recommending this but in my case I filled the area between the ID of the fitting with the Nut hex on it and the partially installed Plasitc Tube with JB Weld. So when I screwed it back in the JB Weld hardened around the Plastic Tube and the inside of the Hex Nut Fitting.

After that I orderd a new Oil Pressure Gauge Line which I have yet to install and that was a lot of years ago now.

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84 300D, 82 Volvo 244Gl Diesel