They would be compensated with the 19th selection in the second round if they choose not to sign the first-round pick from 2018. O'Brien plays college hockey, so his free agency also doesn't begin until mid-August, delaying the compensatory pick until 2024.
Nevertheless, if Philadelphia wants the 2023 pick, an appeals process may take place, allowing O'Brien to become a free agency in June and giving them the opportunity to acquire the second-round pick a year early.