use the open end at a sharp angle, you should only need to apply 20~ ftlbs of so to get it loose, and sine there is a large contact patch its rare that they strip.
Do it right … go to Princess Auto, and get the proper tool - it`s and O2 sensor socket (that has a slite in it for the wire to go throught) that goes onto your 3/8 ratchet.
Now removing the sensor. Attempt to remove - if the sensor comes out with ease - great.
If not you can either use torches - to heat the area to ease the removal. If you do not have a set of torches then - run the engine for about 10 minutes. Be extra careful not to burn your self (wear gloves) and remove it.
If the sensor comes out with the threads - stripped. Do not install the new sensor in, you will damage the sensor internaly. You must tap the sensor hole first. Go to your local garage and ask where you can get a tap (Canaidan tire, Home depot and Princess Auto - does not sell this item - only Snap On, or Mac Tools sell them)