About 2 weeks ago I did the O2 sensor bypass so my check engine light would go off finally. After doing so, my mpg dropped from 30mpg to 22mpg. Also, when I turn my heat on now, the engine revs up an extra 300-500 rpms. Like it's putting a lot of extra strain on the engine. Any idea of what is causing this?

you've answered your own question.

What you lost is fuel mileage will pay for the proper repair. The purpose of the 02 sensor is to control fuel trim or fuel ratio mixture. This has to be done so the cataylitic convertor functions and is not destroyed by overly rich mixtures. Your "repair" is overfueling and cooking the convertor.

ohhh k, I just didn't figure that since I have not seen anybody else with the same problem. Any idea about the heat?

hard to say as i don't have any information on the year or engine.

In defrost and blend modes the hvac system will engage the a/c. The engine needs to compensate for the added load and will increase the idle speed. As the fuel mixtures are all mixed up due to the repair....the ecm may be having to increase idle speed more than usual to avoid a stall. Remove the a/c relay then run the heat and see if there is any change.