If you're investing on a longer-term horizon (a year or more), the monthly futures roll where the USO fund sells the current month's oil contracts (because it doesn't want the actual oil to be delivered) and buys the following month's oil futures (normally at a higher price than what it sold the current month's at) will eat into any returns you get from price appreciation.
To put some numbers to the example, let's say the fund has 100 barrels of oil, and the current month's price is $20, and next month is $21. When it rolls the contracts, it sells 100 barrels for $2000, and buys 95 barrels, with $5 left over.
Fast forward a month. Prices have gone up by $1 for all months oil. It will sell 95 barrels for $22 ($2090, plus the prior month's leftover $5) and buy 91 barrels for $23, with $2 left over.
Fast forward another month. Let's say you owned the entire fund and decided to liquidate it. Prices for your contract have gone up another $1, so you sell your 91 barrels for $24, receiving $2184, and adding the extra $2 in cash you had gives you $2186. That's a 9.3% return in two months.
Compare that to the price of oil as reported in the news - it's the front month contract, so on the face of it, oil has gone from $20 to $24 (20% increase) while you've only made 9.3%.
The numbers are somewhat exaggerated here, and it can work the other way (current month more expensive than forward month) but is uncommon. This is why USO has historically been a bad long-term proxy for the price of oil.
https://personal.vanguard.com/us/funds/snapshot?FundId=0051&...
Note that this is SUPER RISKY investment fund. If you look at the chart it has major swings up and down. The fund is down 20% for 2016 (yikes!). But if you had held it from 2012 -> 2014 you would have gotten a 40% return on your investment.
I like Vanguard funds since they have a low management cost to them and they tend to be a little more conservative in their selection.
Good luck, remember me when you make your million!
Also consider whether you want to invest in oil related companies or in the price of oil itself (which is just as possible).
NB. I have nothing for or against Vanguard funds and have heard a few people supporting them.
Oldest trick in the book: fove out free advice and take a share of the wins, but don't pay a share of the losses ;-)
That's the key underpinning to the stock market. Just ask any broker.