With a yarn over, as long as you are on the very next row, I think you could just add a yarn over to the left needle when you come to the point where it should be, before continuing with the row (knitting it onto the right needle). It would be backwards, but maybe you could switch the needles at that moment to do the YO (as normal), then swap them back to finish with the row? I don't think it would be so easy if you've knitted more than one row beyond the missing YO, but maybe its still possible. This is just a guess, but I hope it helps! Good luck
