What's it mean that the war was "about" oil? Was the U.S. oil industry actively lobbying for the invasion of Iraq? I don't think there's any evidence of that. What would be the material gain from controlling Iraqi oil, except direct benefits for the companies involved? How would it benefit U.S. capital as a whole? The oil industry likes high prices but that hurts almost all other industrial sectors and economic growth and stability overall. It may have been "about" oil in some sense, but was it out of rational calculation or grandiose fantasies of riches and glory?
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Apologies for not having specific references handy but I believe the best argument (as in the most coherent, not necessarily the most correct) put forward to link 'oil', understood broadly, to the invasion of Iraq is the desire of neocons to ensure US dominance well into the 21st century by placing a powerful American military presence at the hub of world oil extraction and shipment.
Doing so, the argument goes (if I'm correctly remembering) would give the US de facto veto power over world energy supplies. Indeed, I recall reading a translated analysis, supposedly from the Chinese government, interpreting the invasion and occupation of Iraq as a blow against Chinese interests inasmuch as the growth of China's economy increases its thirst for, and dependence upon, oil.
So, despite the wariness of the oil industry the geopolitical genuises of the Bush admin and its ideological camp followers, more in love with dreams of glory than market capitalism, convinced of the need for the spartan-ization of American culture (which they believe can be accomplished through perpetual war) and determined to halt American decline -- and idea only they seem to take seriously as a trend, the rest of us being certain American power is quite solid -- have engineered a powerplay to place the US in the seat of decision regardless of whether her economy is strong, middling or weak.
This view sees the invasion as a purely militaristic venture, mostly divorced from the corporate sphere (with the exception of a few firms poised to benefit) and, therefore, a bit of a throwback to more ancient modes of operation.
I believe the growing weariness of sectors of elite opinion with Bushevism (note the cover of the most recent Economist which pinpoints Bush's many weaknesses) suggests there's some truth to this argument.
DRM