Arduino IDE with ATtiny85 error "multiple definition of `__vector_5"
The error message was:


  wiring.c.o [...]: multiple definition of `__vector_5'
  Glove_heater_v1.ino.cpp.o [...]: first defined here


__vector_5 is TIMER0_OVF_vect. So you have one definition of this
ISR in your sketch, and another one in wiring.c. This wiring.c is part
of the Arduino core. As KIIV puts it in a comment, it uses
TIMER0_OVF_vect for timekeeping (millis(), micros() and
delay()).

You have two options for solving this:


you can use the other timer (Timer 1) in your sketch, and leave Timer
0 for the Arduino core
since you are not using the Arduino core anyway, you can get rid of it
by simply defining main() in your sketch instead of setup() and
loop().