Most likely a jump drive (they don't take much), but not a hard drive or SSD. OTG is data in/power out. You just won't get PD (Power Delivery, Samsung's name for Qualcomm's QuickCharge 2.0 - OnePlus uses a different method of fast charge, Warp Charge), but thumb drives, SD cards, etc., don't have batteries, so they don't do PD negotiation anyway. (You could probably even put a little charge into another phone, but the battery is only 3700 mAh in the 7 and 4000 mAh in the Pro, so I wouldn't let that other phone take more than about 1000 mAh of charge. When you get down to about 45%, that's it. [The "drop-dead charging point" - if you want maximum lifespan from the battery - is 40%, Never let it discharge lower than that. If you can't charge it, and don't carry a power bank, turn it off.)