We used it all the time in Alaska during the winter. We would have to add a blanket or pad under the battery and on the oil pan too. It got cold enough that the fluids would xludge if not freeze completely. I'm sure you won't need it in Oklahoma though.
As far as selling it like somebody mentioned, that's not possible unless y-ou want to go through a lot of work. The block heater replaces one of the engine drain plugs. You woulod have to get a new plug, drain all the antifreeze, and swap it for the heater. And the heater would probably get damaged during removal too. My suggestion is to just tuck the cord up out of the way and forget about it.