try a propane/butane hand held blow torch on the o2 and burn any crap off it (don't be afraid to heat it till the tip is red), forcing anything in there air or sandblasting (seen someone say they've done that before) if it wasn't buggered it will be afterwards... generally a poisoned o2 may come good in a car if you can get it running correctly, if it's old and worn out/cracked inside there's nothing you can do except replace it, make sure the outer housing where the wires go in isn't covered in oil, if it is replace it and fix the oil leak, o2 sensor have 2 parts used to measure, 1 in the exhaust and the other in the shielding on the outside, if the outside gets covered in oil it will damage them.
with the blowtorch you can also test them with this, attach a multimeter on volts (mV) setting -ve to the -ve and +ve probe to the sensor output (never attempt to measure resistance this will kill it)
Place it in a vice, heat it up with the flame and try to get the flame going into the holes/veins, the voltage should rise somewhere between 0.7v to 1.2v (depending on the mixture of the butane/propane to air) handy to leave it in the vice till it cools down also.
when the flame is removed it should drop back to zero or to - voltage within 2 - 4 seconds, if it drops slower than this the sensor is possibly lazy and slow
(This is a rough test to check if it's working at all. Not 100% accurate test, if you can't get any reading from it even when it's red hot then it's probably dead)
