There is no user settings in OCPN to adjust the compass
signal. Permanent off line as well as deviation adjustments has to be done before the heading signal arrives into OCPN. Magnetic variation, declination, will however be adjusted inside OCPN by the WMM plugin.
I saw some hints here: BerryIMU Quick Start Guide - ozzmaker.com
If your deviation is already adjusted maybe you can use the NMEA-converter plugin to make a raw off line adjustment but I think it's better to let RPi take care of it?