my mechanic reset my o2 sensor and the engine light stayed of for a while but now it's back on... it is intermittant

what are the causes of this light coming on, anything serious ?

 

depends on the code on if you need to be concerned about it. If it was the bank 2 post cat, no need to rush and fix it unless the cat is bad. If it is for bank 1 precat, then we need the code because it could be caused by a dying sensor or something else