Assuming the pinned together knife:
I'd clean the pivot area out by applying excess oil and letting the oil drain out onto a white paper towel while opening/closing - a lot of "black" debrie may be binding things.
Then cycle open/closed a few dozen times to see if things get easier.
If not, remove all oil - rinse it out with denatured alcohol or clean with DAWN or TIDE. Then, open/close a few dozen times to see if any internal surfaces wear a bit and mate better.
If that doesn't work, there is always spreading the scales at the pivot. This action is tricky as too much will pop the pivot pin.