So in the past I’ve used the apotop wifi thing to connect my iphone to OP-1 to backup files when on holiday etc.
USB OTG cable
You can find them for under $2 each if you look around. No specific apps needed, this is how I back up files to my Galaxy S5.
You’ll see the usual 4 folders, but 2 other folders may show up when connected to the phone (LOST.DIR and ANDROID). Both of those can be deleted and the backup can still be reloaded to the OP1 easily.
I updated the OP1 firmware with this setup too, no need for a laptop or desktop computer.