Is an oil-based roux cooked the same as one made with butter?
A roux is just cooking flour in an equal amount (or thereabouts) of fat. If you've already done it in butter, you already know what you are doing. Butter is tricky because there is some water left in the butter and the fat itself has a low smoke point. After making a roux with butter, another fat is easy.

Butter contains water, but it's not so picky that the small amount of water matters as far as measuring the butter; it's more about compensating for the butter's behavior and not allowing it to burn. 

Do the same thing with any oil as you have with butter, and you will be fine.

If anything, you will find it easier with another fat. Just go roughly 50/50 and take your time.