threads are 5/8-18. you need a die, no tap.
assuming the shank of the oarlock is pretty close to .625 you can run the die down and thread them. if diameter is larger i would not bother, if significantly under the threads will be too shallow.
personally, i have never had a problem with oar locks moving up and down in the towers, unless the oar gets hung up, or i am pushing off the bottom.