The region of an oar that sits in the oarlock is referred to as the oar's sleeve. Historically, oar sleeves were made of leather, which helped to reduce wear of the wood shaft as the oar was used and repeatedly turned from square to feather with every rowing stroke.
On modern fiberglass oars, the sleeves are made of plastic. Plastic sleeves inevitably wear out, and need to be replaced, as illustrated
by these photos from the last time I worked to refurbish rowing oars, way back in 2012 in Texas.
Modern sleeves are also designed to have flat surfaces that hold the oar at very precise angles, helping to ensure that the rowing movement is efficient both when the oar is in the water and when it is out of the water.
That makes sleeve replacement fussy and stressful. Thankfully, the major oar manufacturer provides detailed instructions on how to set everything up correctly. I ensured that the oar blade surface was level across a specific part of the blade, and then ensured the shaft of the oar was also level, lining the oar up across the surface of our basement workbench.

Then I used a rowing "pitch meter" to set the sleeve with a 3-degree pitch.
One part that was confusing to me, however, was whether that needed to be a positive 3-degree pitch or a negative 3-degree pitch. Thankfully, I had another pair of oars nearby, so I could check and confirm a positive 3-degree pitch.
Once the pitch was set, I secured the sleeve by applying a 2-part polyurethane glue.

I completed the process for a pair of oars, one specialized for the starboard side of the boat, the other specialized for the port side of the boat.
Last night while on the verge of falling asleep, a question occurred to me about whether I had correctly set the pitch at a positive three degrees for the second, port-side oar. I will need to recheck it this evening. If I got the port oar backwards, I'll need to take everything apart, pry out the glue, and try again. Sigh.
While I was in the basement, I also sanded the oars to be repainted, and applied the next coat of paint.
It's really important to sand between coats to ensure the paint adheres well.

Even after just one full coat, these oars are already looking so much better than before.

The work will continue.