Got a small glass? Just a reminder lesson coming. Put an inch of water and inch of salad oil in it. I hope you see what I always do. Oil is on top right? It is lighter than the water?
Same with the atomized vapors from either. The draft from the fireplace is catching those from the oil paint first, faster and before water vapors in the room. And it is combusting them. The result is the odor you smell.
Has to happen this way.
If it really bothers you. Close the damper. Keep the fireplace off until you are done. If it has a pilot light, kill that too.
Need a lava lamp by the way? This is where the originals came from and one company still makes them. Take about really cool use of floating oils in heavier solids. Of course I own hundreds.