You may have lost this one but if u have a broker or ur account is with the exchange, u can create a hedge against it, always do that to minimize ur losses. I think u know this.
There's always a reason for the credit. No free money. The reason there was a credit with USO was HTB. Otherwise, it's put/call parity. Could do a box instead of conversions for interest rate play, back in the day of interest rates. It's all about carry costs. Perhaps the easiest way to see it is the div. play. The American style ITM call cannot be properly priced for the div. and the box has a credit because puts can price in div. But the ITM short calls are assigned. Generally speaking.