In Genesis 2:7b What does it mean when it says “and breathed into his
nostrils the breath of life”? is the “breath of life” referring to oxygen
or the spirit/soul?

I would say that, when in doubt, we should let the obvious
meaning stand. Genesis seems to be saying that God made the inanimate
Adam alive, and he began to breathe. Whether or not God put a spirit/soul
into Adam at this point in time is a matter of speculation. One might
even go so far as to say it seems likely that Adam received a soul at this
time, but the Bible does not say this so one would do well to stick with
the obvious.

Many have suggested the possibility that Adam was a previously
evolved intelligent homo sapiens whom God breathed a soul into, making him
into the image of God at this point. I cannot absolutely rule out this
possibility, but again, I feel it is always best, when in doubt, to let
the Bible speak for itself. So if you ask my opinion (and apparently you
did!), I would say that the most obvious interpretation of Genesis 2:7 is
that it is saying that God made Adam alive physically at this point, and
that he began to breathe.

John Oakes