I guess I should probably define what a severance tax is, perhaps that will help with understanding it. A severance tax is a tax imposed by a state government upon corporations that are harvesting the state's natural resources, it's not a lease fee or a royalty. It's a tax.
Do the oil companies have to pay those severance taxes to the state if the oil comes from private land or Federal land?
Sorry, just got back home from Chicago seeing my niece off to college so am late in responding.
Yes, the oil companies have to pay the severance tax on any oil that is removed from the state. Regardless if it is private land, state land or federal land. It's a tax on resources (not just oil, but in this case that is the resource in question) that are removed from the state and sold.
Also, Obama's tax is not properly a windfall tax based upon what has been proposed, which would be taxing profit after the money is made. I'm not sure why taxes like this keep getting called a windfall tax when they are actually excise taxes, which are essentially the same as a severance tax but that can be applied to any good sold and not just a resource. He's not going to go back and tax the profits the oil companies have already made - at least as far as I have been able to discover. He didn't even call it a windfall profit tax in his original speech talking about it, only that taxing the profits (which isn't always a windfall tax as the severance tax is a tax on profit, not income) of the oil companies would help fund some of his energy policies. Most likely, the tax would be similar to the Crude Oil Windfall Profit Tax Act of 1980. If oil companies charge above a set price level, they will incur a tax on the amount charged over the price threshold depending on factors such as the removal price of the oil (cost for getting it to market), inflation (value of the dollar relative to a fixed point, likely when the act is made into law) and state severance taxes imposed on the profits of oil pumped from the various states.