Unfortunately, Rockstar Games does not provide any information about how to get the lost oppressor mark on the company website. The visitors of Rockstar Games community post quite many comments about disappeared oppressor mark. They do not offer any solution on how to get it back either. Please contact the Rockstar Games support team for assistance. You may reach out to them by submitting a ticket at support.rockstargames.com/hc/en-us/requests/new.