That rule has been changed now. PIOs are not longer required to register with police for stay beyond 6 months.
Minors were exempted from police registration anyway..so this change is applicable to adult PIO holders.
OCI card holders don't need to register with police.
See below confirmation -
https://www.hcilondon.in/pressview.php?category_id=1171
PS: also as per old rule, PIOs were required register with police before completion of 180 days if they were planning to stay beyond 180 days. So this could be done even on the 180th day!
Registration with police within 15 days for stay beyond 180 days is applicable to non PIO foreigners.