Hi Masky,
The overcurrent protection monitors the Vds voltage of the FETs such that it compares the value to that stored in OC_ADJ_SET register value. If Vds exceeds this value stored in the register then the overcurrent protection mode of response is initiated based on the OCP_MODE. The various methods of reporting are explained on page 17 and 18 of the DRV8301 datasheet. Keep in mind that there is a 20% tolerance across channels for the Vds trip points and that Rds varies with temperature.
-Raymundo Hernandez-Toscano