It seems to depend on what you have. I have an XT1505 and an XT1527, both US versions bought direct from Motorola. Spec Device says both have OTG, Chainfire's USB Host test utility says only the XT1527 does. I've used a USB drive on this XT1527, I had a Go Phone version of the XT1527 before and I was able to use a USB keyboard (can't find my mouse) and an RT2832 SDR dongle with SDR Touch using a powered USB hub. That Go Phone (and probably most cheap versions) wouldn't tether and Motorola wouldn't give me the unlock code for the bootloader so I returned it. The retail version ($150) tethers fine and I've got the unlock code already.
I keep hoping it's a software/firmware deficiency on the XT1505 that it might be possible to change once it's rooted. At least this one can be rooted, I think. I've had no luck using OTG even with a powered hub. From Linux I can use ADB and fastboot, I can transfer files in and out of it with PCManFM which pops up when I plug it in (Debian). Motorola's Windows USB drivers I've had trouble with. I can tether with both phones over USB to Linux or Windows, the 1527's much faster.