Yarn Overs, the real scoop
I don't actually like the term "yarn over" (yo). It can lead to confusion about how this stitch is worked. In many vintage patterns, particularily from England, they use the term: "yarn in front" or simply "yf."
If a yarn over is worked wrong, it can make smaller holes (which can be a problem in open work) and mistakes down the road, missed stitches and such. Yarn over can imply that the yarn is taken from the back and brought OVER the needle.
A proper yarn over is always worked starting with the yarn in front, between the two knitting needles. No matter if a yarn over is worked between two knits stitches, a purl stitch and a knit stitch or a knit stitch and purl stitch, you always begin with the yarn in front (between the two needles).
Yarn over between a knit stitch and a knit stitch
Step 1: yarn in front
Step 2: slip your needle into the next stitch thru the front loop
Step 3: bring the yarn over the right needle and work the knit stitch.