Even if you don't need millis() function, you can't use TIMER0_OVF_vect because the Arduino Core has defined a Timer0 overflow interrupt (a.k.a. TIMER0_OVF_vect).
If you write multiple ISRs of the same vector, a compile error will occur.
Try using TIMER0_COMPA_vect instead of TIMER0_OVF_vect.
The overflow interrupt must be released and the compare match A interrupt must be enabled.