My tactic for over oiling is to wear my hair up for awhile. After a few days, it wears off a bit. I have dark brown, curly hair with a lot of natural color variation, so once it's up, the length doesn't show oil easily. I don't oil near my scalp if I don't want to wash, so I rarely have to hide the greasy roots look.

I can't go a full month without a wash. Currently, because it's summer and I sweat a lot and gather lots of dust, I'm in the every 3-4 days camp using a homemade soap bar. In Winter, I can sometimes do a week to a week and a half. But I do sweat a lot, even just sitting, and have to wash my scalp even if my roots aren't visibly greasy.