Public service message from someone who went down this road before: it's worth studying up on the futures market. In particular, there's a nasty little thing called "contango" that effectively makes your investment worth less and less every month that you hold it and oil prices don't go up. Think of it as a rental fee for the tanks where they keep the oil held by investors.

The upshot is that you can lose money on oil futures even if prices don't go any lower, and you may lose money even if prices go up, if it takes a long time for that to happen.

tl;dr -- there's no clear way to make an investment that cleanly tracks the price of oil, like there is for the Dow or the S&P 500 or the Japanese Yen.