White smoke from exhaust that disappears = condensation. No worries.
The O2 is before the muffler. So, no.... a crack in it won't cause O2 misreadings. However, a substantial exhaust manifold or post manifold gasket leak could. Fix any of those if you have'em.
Read the troubleshooting guide: http://personal.utulsa.edu/~nathan-b.../3troubles.pdf
Try running some more searches. Not to discourage you from asking questions, but there's a number of reasons you might be having this problem. Many of them have been discussed. Once you've read the troubleshooting guide, use some of the possible trouble components as your search keywords. Moreover, reading what others have been through would give you a number of ideas and things to look for.
I hate to say this, but be careful about some of the diagnostic advice you read around here. Some of it's not so smart. Don't hesitate to ask about it.