The easiest way to do it would be to invest in futures.
Very simply, you’re betting against the future 
Very basic example ahead-
So you would be committing now (at say $1 barrel just to have a nice round number) and promising the trader $1,000 in the future because you’re counting on prices being higher than $1 when that barrel of oil is produced and ready for sale.
Let’s say your bet pays off and, in the future, oil is up to $5/barrel, you’ve made a decent profit. You’ve bought 1,000 futures (barrels) so you owe the trader $1,000 and then you get your $5,000.
However, if prices drop and that oil is only selling at $0.10, you still owe the trader $1,000 but your investment is only worth $100, so you’ve lost money (and may have to find out how to store all of that oil that hasn’t sold because someone invested at $0.08 and sold at $0.09 therefore your oil is overpriced and doesn’t sell but you’ve committed to buying it).
Oil is a really poor investment currently because there’s zero demand, and demand drives value. Certainly, demand (and therefore value) will rise, but nobody knows when that will be.
Also, oil storage is full so it’ll take a while to get to the stage where supply and demand return to normal levels.