> Octals must start with zero and then o/O literals.

No, the o/O is optional (hence in square brackets), only the leading zero is required. All of these are valid octal literals in Go:

0600 (zero six zero zero)

0_600 (zero underscore six zero zero)

0o600 (zero lower-case-letter-o six zero zero)

0O600 (zero upper-case-letter-o six zero zero)

My bad! I was wrong; added a playground demonstration the parsing behavior above.