The o2 sensors would only come on line after warmup (they won't even have any output until they're over 600*F) & it's not so uncommon for the up-stream o2 sensors to be come contaminated (internally from uncured silicones, or externally from grease, anti freeze or other debris).

Checking the vacuum should give some insight to the condition of the cam timing (low or erratic vacuum will indicate an inappropriate intake valve timing)... Checking compression figures may also be a good place to start (low across the board can also indicate cam issues)...