Regarding O2 on a MAT 252:

Our laboratory has been analyzing pure O2 as analyte with a MAT 252 for
more than 3 years and we have incurred no difficulties of the type you
describe. I know at least two other labs that can say the same. The only
effect attributable to O2 that I have encountered has to do with cleaning
the source. I once took the source apart for cleaning (it actually did not
need it, but that is another story). After reassembling and baking we
found that there was some adsorption of O2 that persisted for several
days. It was cured by baking with He flowing for 48 hours. I suspect that
the adsorption following cleaning would not have been such a problem with
a less reactive gas.

-Ed Young
 Earth Sciences
 University of Oxford