O2 Sensor Rusted over

Trying to put a spacer into the O2 sensor on my truck and I am having trouble getting the O2 sensor off. It seems like I am stripping it. I have the snapon tool that fits on the sensor but it just slides around. If I am looking the trick

 

You may have to take it to a shop, or someone with a torch.
.....and a fire extinguisher.

 

Can't imagine you'd get any O2 sensor out that's been on the bottom of a vehicle for even a few years without an oxy-acetylene torch. Well, maybe in the desert...

 

With the corners already rounded off, it’s going to be difficult. I’d use liquid wrench. Repeatedly put some on, let it soak overnight, drive to get it hot. Repeat over and over. Then maybe a pipe wrench. But a pipe wrench will chew it up & maybe damage the sensor. Or worse break the sensor off leaving the stuck piece still stuck. Looking at the exhaust flange fastener, you could end up with a similar problem if it strips too.

A 7/8 multi spline swivel wrench might work if the sensor connector fits through the hole.