YO is the abbreviation used for yarn over, and is used generically in instructions. There are a couple of ways to work a yarn over. You may find that you want to work the yarn over in front of a knit and purl differently to get the same size stitches.
Yarn Over – before a knit
Bring yarn forward as if to purl then going over the right needle, return yarn to the back of work to work the next stitch.
Yarn Over – before a purl
Bring the yarn backwards between the needles then over the top of the right needle into position to work a purl stitch.
Yarn Over – smaller YO before a purl stitch
Bring yarn over needle to front and purl next stitch using finger to hold yarn on right needle. This makes a smaller opening.
Yarn Over – larger YO before a purl stitch
Bring yarn forward between needles as to purl then going over right needle, bring it forward again and purl the next stitch. This makes a larger opening.