With Oil futures trading at negative prices yesterday, what happens to ETFs like USO that hold these contracts? If one buys such an ETF, is it possible to lose more than the price at which it is purchased, i.e. could it have a negative price, which the investor would be on the hook for?
No, because the fundamental cause of the negative oil price (limited storage of the physical good) does not exist for ETF. There is no cost to holding an ETF, so there is no reason that you would need to pay someone to take it (if it was worthless you'd just hold it)
Now the ETF itself could actually have no NAV (due to leverage and/or losing money on futures contracts) but the value of the ETF itself would not go negative - it would be similar to holding the stock of a bankrupt company.