The Knights have convened a subcommittee of five delegates to select their coach for 2026 after announcing last week that O'Brien would step down at the end of his sixth season.
The decision came as the Knights fell from finalists last season to wooden spoon candidates in 2025.
Former Gold Coast coach Justin Holbrook, current Knights assistant Blake Green and ex-Cronulla boss John Morris are among those to have been linked with the vacancy.
Hagan joins Knights CEO Peter Parr and chairman Geoff Coburn on the subcommittee, alongside Parr's successor as football boss Chris James and board member Tony Price.
In 2001, Hagan led the Knights to a 30-24 win over Parramatta in the decider, rated one of the NRL era's greatest grand-final upsets.
Hagan coached the Eels for two seasons after leaving the Knights in 2006, and was in charge of Queensland for two State of Origin series.
He also served on the Australian coaching staff under former boss Mal Meninga.
O'Brien will coach his final game against the Eels on Sunday afternoon.
If Gold Coast beat Wests Tigers on Saturday, the Knights must defeat the Eels to avoid their first wooden spoon since 2017.
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