Look on the side of the engine. It will tell you what the engine is.. OS 40, OS 46, whatever. then go to the OS web site and download the manual for that engine and read up on what fuel it requires.

Most OS engines will run just fine on 15% with at least 18% oil blend. The important part of this is the oil blend. Make sure the oil you use is at least 18% and, if it has a higher nitro content, then as Ken said, run it rich until you are finished with that fuel and then get the recommended fuel. Trying to blend your fuel with something else to reduce the nitro content is just a pain and, with the quantity you probably have, most likely less than a gallon, it just is not worth it and you probably will never get it right anyway.

Finish up with what you have, running it rich, and then get something more suitable.

CGr.