I've done several and one was easy, one was difficult, depending on age, sand, handle condition. I turned new handles for a brand new pair recently and when I took them apart the silicone was still wet, simply removed the screw and pulled apart with moderate effort and twisting - 5-min tops. They were very easy.
Prior to that I removed some from a 6-7 year old pair and I tried a lot of things in vain (prying, twisting, etc...). What ultimately worked was to take a piece of pipe (I threaded two pipe clamps I had together ended up about 2' longer than the oar.) and set the pipe on something hard and then drove the handle out (I turned it vertically and used the oar as a slide hammer). It took a bit of effort to get the handles off... I would suggest steel pipe with a pipe cap on the end going into the oar.
My suspicion would be that unless they're new they won't just slide right out, you'll have to drive them out. Also there should be a foam plug just above the blade set hole...it will likely get destroyed if you have to drive the handles out, so be prepared with some ethafoam or other closed cell foam to replace it. I stuffed an 8' piece of 1.5" pipe insulation into mine so they wouldn't ever sink, even if filled with water.
Oh, yeah at the factory they seal the handle end with silicone, so you'll want to cut that with a box knife. You should feel the knife cutting wood once you've gotten all the way through the silicone. It will take several passes...
On a comical note this post was really hard to write without being full of sexual innuendoes... too many shafts, holes and pounding...
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae