I just got my first bucket of oxyclean today. It doesn't have directions for cleaning beer making equipment, but it does give a few examples. On the bucket, it says 1/2 to 1 scoop per gallon for soaking (with a picture of a shirt soaking), and 1/4-1 scoop per gallon for outdoor use (it gives decks as an example).
My scooper has markings that show a full scoop is four onces. I went with 2.5 scoops for each carboy/fermenter, then I used a bit more at the recommended levels to try it one some carpet stains and a messy stove. The 3.5lb bucket is almost gone.