On an AVR (ATtiny2313), which is serviced first when TIMER0_OVF and TIMER0_COMPA are due at the same time?
See the datasheet for the ATtiny2313: http://www.atmel.com/dyn/resources/prod_documents/doc2543.PDF

The priority of interrupts is determined by the order of the interrupt vectors.  See page 46 for the list.  The lower the number, the higher the priority.  Since Timer/Counter0 Overflow has a lower number (7) than Timer/Counter0 Compare Match A (14), the overflow ISR will run first.