2003 Xterra 6cyl 4wd.....check engine light came on last night. Checked all the fluids....little bit low on antifreeze...topped it off. Autozone checked the codes 0154 (o2 sensor) and 0327 (knock sensor)...Should I take it to the dealer or is this something I can do with limited tools and freezing garage?

With the right wrench O2 sensors are not bad to do. the knock sensor does not trip the CEL, unless you are SC then it will put it into bypass.