2002 pontiac grand am 4 cylinder sedan cant pass emissions p0141 code 02 sensor heater cicuit malfuntion [bank 1]po300 random/multiple cylinder misfire detected,changed oxygen censor under hood still will not pass emissions and check engine light still on were do i go from here.

Not enough information so i will cover many possibilities:

new spark plugs would be a good place to start .. then replace the ignition wires, clean the coil insulator .. clean or replace the distributor cap .. when you replaced the O2 sensor that should have eliminated the "heater circuit" problem (the O2 sensor has a built in heater that gets it up to working temprature faster at warm up) .. .. if the wiring and connectors to the O2 sensor are ok then that fault code should go away .. multiple misfires will keep the trouble light on .. that could also be caused by a sticking or dirty injector or dirty throttle body.. you can use a good injector cleaner for that .also look for a cracked vacuum hose ... it would be good to know the present fault code since it should have changed once you installed the new O2 sensor .. there may be two O2 sensors (one later in the exhaust system) ..it sounds like you got the right one but make sure ..it may take some time for the computer to adjust to the new O2 sensor and for the injector cleaner to work .. make sure new plugs are gapped properly and use an "antiseize" compound on the threads .. that reduces the probability of the plugs sticking in their sockets .. on aluminum heads the plugs want to seize (or weld) in place after a long time. .. that makes them really hard to get out later .. just a tiny amount of antiseize compound on the theads helps a lot .. be careful not to overrtighten plugs in aluminum heads ..
if your engine is badly missing on one cylinder (you should easily detect that) then a sticking injector would be suspect .. there are some simple ways to get it working again if thats the case ..

Change the mass air flow meter.