How can I easily soothe a baby's diaper rash with OTC creams?

Find the right one. Sometimes you have to try a few different kinds to find one that works well for your baby. Desitin, (zinc oxide diaper cream) a & d ointment, zinc oxide paste, plain old vaseline, and others are possibilities. In nursing homes, products called barrier cream and butt frosting are used for similar problems of incontinent adults. These might be useful for a baby also.
Exactly. Am not sure what the question is. You have described how to soothe the diaper rash with one of numerous products available otc.