where/when to add OxyClean
You can add OxyClean either to the dispenser or right into the drum, before you start the machine. Anything that is placed in the detergent portion of the dispenser drains directly to the tub anyway, it isn't released later in the cycle at a special time (unlike say the fabric softener section of the dispenser). If you place liquid detergent in the dispenser (which I would not recommend, as eventually it gums up the dispenser), it drains as soon as you close the dispenser door, even before the machine is turned on. Powder detergent will partly drain to the tub, and when the machine is turned on, a water flow through the dispenser begins to wash remaining powder or liquid into the tub before the main wash cycle begins (i.e as the machine is filling with water for the first wash cycle).
So you can add the OxyClean and detergent right into the tub, put one ingredient in the dispenser and the other in the tub, or put both in the dispenser....however if you use liquid HE detergent, avoid the dispenser to keep it clean, just put it right into the drum.
I use a small plastic measuring cup that comes with the large "spigot" bottles of detergent, fill that about 1/4 full with liquid or powder HE detergent) and just place it in the drum with the clothes...at the end of the wash, the cup is rinsed clean and ready for next use. In Europe people often use dispenser balls, basically a small plastic ball that's hollow with a hole into which you pour the detergent, then place it with the wash, the advantage over the cup being there are no plastic edges to snap and possible damage clothing, though it's never happened to me in two years of FL use. I don't have a laundry sink, so if I poured the liquid from the cup into the drum, I would have to walk the cup into the kitchen to rinse it out. Also, because I place the cup upright within or on top of the clothes pile, the detergent doesn't touch the clothes until the washer has already filled with water and begins to agitate.
If you have hard water, consider a few tbsp of borax along with the detergent and OxyClean. I have hard water where I live and Borax produces cleaner rinses (no soap suds left on window at end of final spin) and whiter whites.