Both loaders do register their USB libraries for use by the game too, so there should not be a problem of the YaBASIC not having access to the USB hardware.
Originally Posted by junebug
It's most probably either because the YaBASIC USB keyboard driver is incompatible with the USB drivers used by OPL and PS2ESDL, or has compatibility issues with your USB keyboard.
Have you tried to re-plug in your USB keyboard after YaBASIC starts up?
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit