I'm not sure if OxiClean would be good for DWR fabrics.
I know that it would not help the actual cleanliness if regular detergent isn't helping. OxiClean is a bicarbonate bound peroxide which produces H2O2 in water. The oxidizing reaction of hydrogen peroxide "bleaches" the colour from a stain, even though the offending material is still there.
When DWR material gets dirt in the fabric pores, the mechanical abrasion wears down the Teflon coating on the fibers, reducing the water repellency of the material. The dirt also clogs the pores of the fabric, reducing the breathability.
OxiClean might make it look cleaner, but functionally it would still suffer.