I'm not sure about how to get the oil down from the scalp but for me the longer I go without washing the more oily my hair tends to get, like you though I'm co-washing almost every day due to my workout schedule. For me having such long hair it's almost impossible for the natural oils to reach my ends where I need it the most so sometimes I just add a bit more leave in or oil to my ends.
I like doing an oil treatment before I wash but I don't do them very much anymore because I do need to use a sulfate free shampoo to remove the oil and I have to shampoo two to three times to get it all out so I don't do that quite so much. If I don't wash the oil out it leaves my curls a bit limp and greasy which I don't like at all. Some people can actually rinse out most of the oil using only conditioner but this just doesn't work for me.