OG&E says don't, but doesn't say anything about legality. I went ahead and had a transfer switch installed when I redid my electrical panel a few years back. Makes things so much easier.Why wouldn’t it be? Your generating your own electricity with your own generator and feeding it into your house. My FIL did this with the back feed going into the 220 outlet for the dryer. The dryer vent was on an outside wall and he’d feed the cord in through it.