I recently got blue under glow put on under neath my car for Christmas. I was just wondering if it's actually illegal to drive around on the street in Oklahoma with it on?

In Florida, Arkansas, and Ohio it is perfectly legal as long as the light was not blaring into other car's windows or causing a distraction. Oklahoma may be different. You should check with your local police to be sure so you won't get a ticket for this. Also if you plan on traveling out of the state you may not wish to have the underside lit up otherwise you could be pulled over and fined.

Did you ask the installer. I would suspect they wouldn't have installed it if it was illegal. They could be held liable if it was.

It's not up to the installers to be held liable if it is legal or not. I could cite several instances where items are sold and the sellers are not held liable. Guns and gunshops are a prime example. If install somethng for someone it is not up to the installer to insist the item be used legally or illegally. Granted, honest mechanics who do install things like these lights won't install them if they know they are illegal. But what about Mr. Shade Tree Mechanic with no shop? He can install anything he wants. Or the owner of the car can install them himself. OP didn't state who installed the lights under his car.

Same goes for tint on car windows. Some states do not allow the really dark tint but car owners get it anyway and deal with the tickets and fines on their own knowing full well the tint exceeds legal limits. Or they install it themselves.

I tried ticketing a few here in GA some years back, ( GA has * or had* a vague blue light law) Here if the light can not be confused with a emergancy vechile then it can not be.

Had a set of blue lights on the windshild wipers that gave a "blink" effect when they went back and forth, I got that ticket to stick.

Looking back, most have been some boring days

None of the examples you give hold water here. Guns can be sold legally and used illegally. In NY an installer who installs tint greater than the legal limit can be fined.

In the case of the OP, the type of underglow referred to would have to be know by the installer whether it was street legal or not. It would be one thing if the underglow was sold to the OP and installed by him, but if the installer installed it, they could be held liable.