The output compare registers are registers to set the duty ratio for PWM output.

The output compare registers are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.

#define OCR0A _SFR_IO8(0x27)
#define OCR0B _SFR_IO8(0x28)
#define OCR1A _SFR_MEM16(0x88)
#define OCR1B _SFR_MEM16(0x8A)
#define OCR2A _SFR_MEM8(0xB3)
#define OCR2B _SFR_MEM8(0xB4)

It calls _SFR_IO8() _SFR_MEM16() and _SFR_MEM8().