You can buy USO if you have a brokerage account. Be aware that it does not track the futures/cash price perfectly as it has to pay the contango.
USO is going to have a harder time of it tracking oil. The ETF is running out of investment vehicle options.
And the roll slippage is not insignificant.
They were 30% of the open interest in June, which has now expired, but CME told them to roll and to get further along the curb to stop another negative incident.