According to the authors of "When Bad Things Happen to Good Knitters", a missed yarnover discovered in the previous row of knitting can be fixed thus:
"...pull up the thread between the stitches where the yarnover should have been and knit into it."
I have tried this, and the resulting yarnover was very tight, making the hole a bit smaller than it should have been. But in my case, it was acceptable. The tinking option was not.