Many thanks to all. Yes, the OFO global app does take credit cards and does work on the Mainland.

A few pointers for anyone else who wants to try OFO:

-- On Android at least, you have to download the global app from Google Play (not available in China without a VPN) and the app is only available in certain regions of Google World. You may have to try some workarounds with Google Play if you're in the wrong region and a search for "OFO" doesn't turn up the app.

-- After adding and verifying your card, you have to go into the cards page and make sure your card has a green check alongside it as the default payment card, even if it's the only card there. Otherwise you'll go around in circles trying to actually make a payment.

-- The refundable deposit is 199RMB on the global app, compared with 99RMB on the Chinese app. But on the global app, there's no need to register your identity with a passport and picture: just pay and go.

-- The app is buggy. And the system is often busy when you try to unlock a bike. That is, it's not very reliable for now.

-- As you'd expect, the bikes are sturdy, heavy creatures and not much fun to ride. Problems are common, so look for a good bike. The OFO bikes do have the great saving grace of an adjustable seat.

Finally, I just read that Mobike and OFO are discussing a merger -- before not after they kill off the competition presumably -- so rates will probably go up at some point. Hard to see how they make money now. (I've taken several rides and haven't been charged anything yet, either.)