2002 300M CEL codes

Car-2002 300M 3.5L HO-My engine has been running rough for awhile now at curb idle...the RPM's don't move very much just the engine feels like it is misfiring. Then 2 wks ago the CEL came on with a P0300 code-Multiple engine misfire. So I reset the code by disconnecting the battery for 30 min. Codes were gone, but the engine was still running rough. Then 1 wk ago I got a P0133 code - 02 sensor 1/1 slow response. The CEL went out on it's own after a few starts. Now today with the CEL back on and the engine running rougher I also get the code P0172 Fuel system 1/1 Rich along with the P0133 code. My worry is that if I replace the O2 sensor this will not solve my problem and that the problem is a bigger one which caused the initial P0300 fault and is now causing the O2 sensor to crap out. Any ideas?

the upstream o2 controls how much fuel is injected into the system. Replace the 02 to start and do not disconnect the batt. That is the worst thing  u can do as for as trying to fig out whats wrong and if it fixed it.
If u correct the issue the computer will rerun the same test that turned on the CEL and if it passes 3 times under the same conditions as the CEL was turned on the computer will turn it off for u and U know its fixed with out guessing.  So  replace the 02 sensor  and for good measure replace the plugs with champion plugs factory plugs  nothing else.

Thanks for the reply! I have bought a new O2 sensor and will replace it....I will replace the driver side soon as well just to be sure. I'll post back here with the results.