Private citizens pretty much cannot buy actual stock in crude oil. The best one can do is trade in oil futures, but you'll need to hold a minimum of $10,000 liquidity in your account to do that with most brokers and be prepared to break even or get wiped out should oil continue to stay low or go lower. The fear there is that as soon as global oil demand rises again, Russia and the Saudis will reinstate their price war to secure contracts and corner the market. This would keep oil prices low if not lower.
The second best thing one can do is a ton of research on companies' financial health and take a substantial risk buying stock in smaller oil exploration and mining companies whose stock has tanked and hope the companies survive long enough for their stocks to rise.
Neither of these two options will allow you to own actual stock in oil. You'll either own stock in speculating what oil will be worth in the future, or you will own stock in exploration/mining companies.
Investing in big oil corporations is a nonstarter since they aren't hurting much at the moment. For instance, Chevron was at about $90/share two weeks ago prior to oil diving below $0. Their stock decreased to $82/share the day oil crashed, and as of closing Friday, stock was back to about $88/share. Same pattern goes for Exxon, Philips 66, and the other major US corporations. There's no profit to be made there due to the oil crash.
As for oil ETFs, they are a ponzi scheme during volatility since they don't hold stock in actual crude oil but instead track futures indexes that are months behind what is currently happening. Those ETFs work pretty well when oil is stable, but when it's volatile they are junk since it takes months for their true value to show in the market. People buying ETFs now thinking they're going to cut a fat hog in four months will be shocked in four months when those ETFs finally catch up to the April crash and the ETF stock is worthless while oil prices might be much higher at the same time.
Trading in actual crude oil is a game only billionaires, millionaires, and corporations who have the capital and ability to store physical oil can play. The rest of us get to dream about how cool it would be to buy thousands of dollars of oil at $0.01/barrel, store it somewhere, sit on it for a couple years, and sell it at $60/barrel. In reality, the best we can do is buy a few 55 gallon drums of gas for $65/each, put them in our residential garage/shop for later use, and hope they don't blow up our house and cars or that the neighbor doesn't call the cops.
We then wait until the price of refined oil decreases, buy out all our local parts stores of our preferred viscosity, and have enough oil for three lifetimes of oil changes. In either case, we'll be those weirdos standing outside a gas station in four years trying to sell quarts of oil and gallons of rotten gas at a discount out of the back of a 1949 F1 panel truck.