How to fix the Upper engine oil cooler fitting leaks water on 95 Chevy Tahoe?

From your question, I think you are refering to the lines going from the oil pan to the Radiator. Those lines are pressurized, and the best, really the only way, to fix them is to replace the lines. Now verify that it IS the Oil Cooler lines, because the only thing that should be running through those lines is motor oil. If they are leaking water, that is an indication that the connection on the radiator itself is bad, and the radiator must be replaced before water contamination is introduced to the motor oil, which would destroy the engine rather quickly.

Hope this helps!