No, there's not really much you can do at that point since you'd be cleaning out any content there you'd need to redo the phone back to a functional state. The only types of ROMs you can flash directly in fastboot mode are pretty much the factory CyanogenOS ones from what I can tell, those are the only ones provided as actual partition images, the rest are designed to be installed through recovery, even OxygenOS comes in a recovery flashable form (the last one for the OPO, 2.1.4).
Well there's one possible way: using an OTG USB stick with the ROM files on that, it might work but I can't say for certain it will. You could do the full wipe including the internal storage, use the file manager in TWRP to copy the ROM from the OTG USB stick back to internal storage then flash, maybe.