Did you forget to make a yarn over (YO) in the row below? No problem, and no need to rip back your knitting. Your forgotten yarn-over can be added in the next row.
Step 1:Work to the exact spot where the YO is missing. At this point, in the row below the one you're in, there will be a stitch tail (tail = yarn which connects two stitches). This is the tail which, if you hadn't forgotten to lift it over the needle, would have been the YO.
With your right needle, lift this tail (blue in below illustration) in the direction of the red arrow, and put it on your left (holding) needle.
 |
| Step 1: find the tail (blue) and put it on the holding needle, following the direction of the red arrow. |
Result: you've made a mini-YO, just where the original YO should have been. Next, work into the mini-YO in the ordinary way, as shown in the below illustrations. Make sure which arm ought to be forward by comparing to the other YO's in your work and to the illustrations below.
 |
| Step 2: This newly-made YO is now on the holding (left) needle, with its left arm forward. Knit into it following the path of the red arrow. |
 |
| In contrast to the previous illustration, here the newly-made yarn-over lays on the holding (left) needle with its right arm forward. Knit into this new YO by following the path of the red arrow. |
The finishing step takes place once you are a few rows or rounds past the error. Using the tip of a knitting needle, ease some spare yarn from the neighboring stitches into the mini-YO. This enlarges the hole to better match other YO's which may appear in your work. This "I forgot" YO may look smaller to you, but no one else will ever notice (and not even yourself, once a few days have passed).
--TK
PS: If these illustrations look familiar, they are, being highly related to a previous post about knitted increases. Part of that post illustrates how to go back and fix a missing twist-base increase, often called an M1. I mentioned (but didn't illustrate) that a missing YO could be added after-the-fact in almost the same way. Well, oops--my lovely readers let me know the lack of an illo was an oversight. All fixed now, though.