Leaks from the oil tank cap are usually due to a dirty seal, a cross-threaded cap, or a damaged O-ring.
First, clean the area thoroughly. Wipe down the cap and the filler neck with a cloth to remove any sawdust or dirt. The sealing surfaces need to be clean. Check the inside of the cap for its rubber O-ring or gasket. Make sure it's present, flexible, and not cracked or deformed.
Now, reseat the cap. Align it carefully with the threads on the tank. Turn it clockwise by hand until you feel it seat, then tighten it firmly but don't over-tighten, as that can damage the threads or the seal. Checking the fuel cap regularly for tightness is good practice for the oil cap too.
If that didn't work: The leak might not be from the cap. Check the tank itself for cracks, especially if the saw was dropped. Also, check the oil outlet hole on the saw body where the bar mounts. Debris can clog the oilway, causing pressure to build and force oil past seals. Cleaning the oil inlet hole, oilway, and bar groove regularly is part of standard maintenance.
Heads up: Use only proper chain oil or a recommended substitute. Using motor oil or other lubricants can damage the oil pump seals and cause leaks. If the cap seal is damaged, you'll need to get a replacement cap or O-ring from a STIHL dealer.