The last 2.1 oil cooler o-ring I changed I did with the double nut method...there was limited room to work, but it did work and I didn't have to drain my coolant....and yes, the old o-ring was hard as a rock, no wonder it was leaking.

Worth a try if you can get a second nut (any VW shop should have extra nuts, since they used similar coolers on other VW engines).

Once you remove the threaded pipe, you can insert the o-ring between the cooler and block then re-assemble by reversing the procedure....now if the coolant hoses are old and hard then you gotta drain coolant anyway and change those little hoses...

As always, YMMV.