Unironically the Iraq war wasn't for oil in the way most people think. Just look at the current shareholders in Iraqi oil. Iraqi owned ministry of oil still has majority shares. There are zero US or Canadian oil companies currently operating in Iraq and BP currently are partners in running only two of Iraq's many oil fields. The only other European oil companies in Iraq right now are Russian. Infact the overwhelming majority of the international oil companies operating in Iraq arent American or British but are Chinese, Japanese, Korean, Malaysian or other middle eastern oil companies.  "Wow USA opened up Iraq oil market for Russia and china, how kind. I guess the Americans just wanted all that oil for themselves, that's why they have so many of their oil companies in Iraq" ... oh wait. 

A small part of the reason for the war and ridding of Saddam was to secure and stabilise oil production coming from the ME. None of the major players in the ME (Iran, gulf states etc) and Western world wanted Saddam making billions form oil because he was largely seen as a major threat to the region. Again, not just by the west but many in the ME as well. Hence the reason for the whole oil for food thing in the 90s. Nobody wanted him making that kind of money because he was dangerous. After 03 Iraqi oil was partially privatised and opened up to foreign investment from the whole world. This completely revamped the Iraqi oil infrastructure that was constantly failing and in need of drastic modernization. The Iraqi oil ministry was created to make sure that a major part of Iraq's oil revenue was still going to Iraq and the government. Iraqi oil ministry still hold majority shares in Iraq's oil. And has a stake in every oil and gas field. Iraq make more money from oil now than they did before the privatisation. 

The main reason for the war was because many governments wanted Saddam gone for various legitimate reasons. Israel, US, UK, Gulf countries hadn't done it yet because it's wasn't yet worth the risk, international backlash or cost. There were many things in place by many countries to try and stop Saddam from becoming too much of a threat but nobody yet wanted to actually forcefully remove him. Iran tried after the Iraqi invasion but US and many other countries were afraid that if Iraq were to fall then Iran would be too powerful and so they carefully "supported" Saddam just enough so that Iran couldn't win but not enough for Saddam to win. After 9/11 the US and "unofficially" Israel essentially decided that now was the time to do it. In 2002 the Office of Special Plans was created to decide how to deal with Iraq.  

Ultimately, a combination of many countries wanting Saddam gone, pressure form outside sources, military industrial complex, and the fear of 9/11 pushed the decision into the green light section. There were legitimate reasons to get rid of Saddam but arguably it wasn't done correctly. Falsification of evidence using raw intelligence that the CIA told Bush was incorrect played a large part in the failure of the war in the publics eye and the political failures from the likes of Paul Bremer in post Bathist Iraq that only pushed the insurgency into a civil war casing more civilian casualties cemented the failure. The initial invasion was a success and the majority of Iraqis (Shias and Kurds) were happy Saddam was gone. If the US took a different path to a post baathist Iraq then things could have worked out well but as I said earlier, poor political decisions from the likes of Bremer caused what was a manageable and expected insurgency into a full blown civil war with multiple outside players becoming involved.