I believe very strongly that the belief that the US invasion of Iraq was driven either solely or in large part by oil interests is not correct.  This belief usually rests on one of three claims, two of which are relatively easy to address and one of which is more difficult but ultimately still wrong.

The three claims are:
Government-wide Oil Conspiracy Theory: The United States invaded Iraq in order to take Iraq's oil for itself.
Ruling Clique Oil Conspiracy Theory: The United States was driven to war by a small group of politicians with financial interests in oil companies, chief among them former Vice President Dick Cheney, for personal benefit.
Strategic Control Over Oil Theory:  The United States' invasion of Iraq was conducted in large part to secure oil supplies for the US and friendly countries in opposition to other creeping strategic threats.
I will address each of these in order.

Claim (1) is that the United States invaded Iraq in order to take Iraq's oil.  However, this is manifestly not what occurred.  Instead, the United States allowed a reconstituted government of Iraq to sell oil on the world market.  What Iraqi oil made it to the United States or to US forces in Iraq was purchased at fair market price, with revenues from the crude accruing to the Iraqi government, such as it was.  Were claim (1) true, the United States would have stolen the oil, refusing to pay fair market value for it, or at least extracted significant concessions in the form of discounts.  Instead, the US invaded and then its citizens proceeded to pay the same price they would have before - in fact, probably a higher price, considering that the 2003 invasion caused a rise in the price of oil later exacerbated by increasing demand from developing economies.

Claim (2) is that a ruling clique with financial interests in oil resources used the United States military and government as a proxy for private business interests.  While I do not deny that there was a significant amount of US business interest in postwar Iraq and that there may have been a significant amount of personal enrichment going on, I find claims that business interests drove the war to be outlandish.  Dick Cheney, for example, had no profit-linked financial ties to Halliburton or its subsidiary KBR in 2003.  Additionally it also fails the crucial test of magnitude.  Over the ten years of war that collectively cost the American people thousands of lives (not including tens, perhaps hundreds of thousands of Iraqis) and trillions of dollars (including interest), approximately $138 billion was contracted to private companies, much of which was providing services to American military personnel.  While this is a huge amount, it is only about 10 percent of the nominal dollar cost (i.e., excluding future interest payments) of the Iraq war, which came to $1.4 trillion.  A government driven by war profiteers probably should have been able to divert more to the cause of seizing oil assets, no?  I conclude that this theory fails to hold up to scrutiny.

Claim (3) is that the United States invaded Iraq in full or in part because it wanted to secure supplies of oil for its allies and in opposition to its strategic threats.  This is more plausible in part because US strategic interests in the Middle East have been, loosely, defined by several major priorities (bearing in mind that I am not commenting on my own personal approval of these policies):
Israel
Oil interests
Cold War alignment (to ca. 1990)
Islamic extremist terrorism (from 1998, intensifying greatly after 2001)
Nuclear Nonproliferation (Israel excepted)
Most US commitments to the region flow from these strategic priorities.  Claim 3 rests on the specific premise that the US invasion of Iraq was intended to enhance US oil supply security.  On its face, this seems plausible - after all, oil is one of the major US strategic interests in the region.  However, upon closer examination this, too, falls apart for a number of reasons.

The first reason was that in 2003 oil was not a critical strategic priority for the United States.  This would change by 2005, but in 2003 the United States had just been through 12 sustained years of some of the lowest (inflation-adjusted) oil prices in history.  Oil was undoubtedly important, but its supply was not in question, nor was the future outlook at the time anything but rosy - the myopia of most oil analysts at work.

Moreover, when push came to shove, the United States was in an ideal strategic position.  There were and still are a host of oil suppliers to the United States that had close relationships that could not be broken without a major strategic realignment.  Notably, these countries included Canada, Mexico, Trinidad, Venezuela, Nigeria, Turkey (important as a transportation hub), Saudi Arabia, Bahrein, Qatar, and Kuwait.  Venezuela was so tied to the United States that even after the election of the anti-American Hugo Chavez, Petroleos de Venezuela S.A., the state oil company, continued to own and supply the Citgo refining and distribution company into 2010 (having previously invested millions into equipment in their refineries to process heavy Venezuelan crude in order to ensure that it would always have a US market).  Saudi Arabia was the host to thousands of American air force and navy personnel enforcing the Iraqi no-fly zone and guarding against another presumptive push on Kuwait or, possibly, any incursion from Iran.  Oil security qua security was not a major issue in 2003.

Moreover, Iraq under Saddam Hussein served several useful purposes that satisfied presumptive US oil aims:
As a regional power, it was a counterweight to potential Iranian efforts to affect the Strait of Hormuz.
Iraq was totally cowed in terms of territorial expansion due to the beating it received in 1991 and constant reminders (one of which, in early 2001, was on of George W. Bush's first military actions) through enforcement of the Iraqi no-fly zone
Iraq was itself an oil producer under the thumb of internationally driven but US-led sanctions.
To say even more, even without the benefit of hindsight, attacking Iraq without a coherent plan for reconstruction was the surest way to destroy oil output rather than restore it to ensure cheap supplies for the US or US allies.

Another claim frequently made with point (3) is that the invasion of Iraq was to counterbalance China.  However, it is difficult to see how one can make this case without the benefit of hindsight.  The China of 2003 was not perceived as a strategic threat to the United States worthy of a global response except by the most hawkish of US security policy nerds.  The Chinese, regionally, and in large part in the US, were viewed much more benignly than they are today in US politics.  On the contrary, China's rise generated a good deal of goodwill both regionally and abroad because of its effect in lifting all of the regional economies.  This did not change until later in the decade when it became increasingly assertive in enforcing territorial claims.  To claim that China was one of the US's top strategic priorities in 2003 is to ignore the national establishment's - and nation as a whole's - obsession with Islamic extremism following the emotive September 11th attacks.

Finally, another strong case against point (3) is that for what the United States spent in Iraq, it could have bought literally billions of barrels of crude, enough to cover the totality of imports for more than a decade.  While this was advertised as war on the cheap by Defense Secretary Rumsfeld, it was still not at all cheap in absolute terms, nor was continuing.  As Nursultan Nazarbayev (President of Kazakhstan) pointed out (in a conversation related from leaked diplomatic cables), the United States could have bought all of the oil produced from Iraq for 50 years for what it ultimately spent.

All of this would, of course, be merely naysaying an otherwise plausible hypothesis if there were no alternative explanations for the war in Iraq.  Strong refutations can only exist in the presence of a countercase that explains the facts equally well or better.  In my opinion, a strong countercase can be made for US actions in 2003 using some combination of liberal international idealism, a strong anti-Islamic terrorist bent, a strongly idealistic desire for international democratization, flawed intelligence, groupthink, the official explanation of WMDs, and many others.

As for what the ultimate cause is...  the 2003 war in Iraq and its long, bloody aftermath is one of the defining parts of my experience growing up and my young adulthood.  I suspect that my takeaway from it will stay with me for the rest of my life.  I do feel I know why the leaders of the United States sent the country to war, nor do I know for certain what, if anything, was accomplished... but I do know that it was not about oil, except in the most peripheral sense.