If you buy one of these oil futures then you are required to receive a certain amount of a certain kind of crude oil at Cushing, Oklahoma on a certain date after expiration. You don’t pay anything at that time as you already paid for the oil by buying the future. Likewise if you sell a future then you are required to deliver a certain amount of oil to Cushing, Oklahoma, and you were already paid when you sold the future. In this sense it is quite reasonable to hold a negative position in a future (you can have a negative position in a stock too but this requires selling short—you need to first borrow the stock from someone else).
One thing to note is that futures are fungible: because the contract doesn’t say who you have to give oil to, you can sell a future at one price and buy it at another price and the two sides of the contract cancel out and you have no obligation to move any physical oil around. Most market participants cannot take or deliver physical oil and so they will make sure they get their position to 0 before the futures expire.
It’s a weird question as to when futures are created. The invariant is that the total of everyone’s position is 0, so every obligation to deliver oil is matched by an obligation to take delivery, and you can then say that the number of created contracts is equal to the sum of all the positive balances, and each trade either creates or destroys futures depending on how the balances change.