Albion were without Richard Carpenter for Peter Taylor's first match in charge at Withdean this weekend.
The midfielder recovered from a calf injury which forced him off towards the end of last Saturday's 2-1 win at Huddersfield but he was struck down in training on Thursday by suspected glandular fever.
The Seagulls had a ready made replacement in skipper Paul Rogers.
German striker Dirk Lehmann was back on the bench in the absence of Carpenter.
Taylor relied on caretakers Bob Booker and Martin Hinshelwood to pick the team, having been involved in his first training session with the players only yesterday.
Ritchie made a trio of changes to the Oldham team which suffered a first home defeat against Bristol City last Saturday.
Scott McNiven and John Sheridan returned from suspensions to replace Paul Rickers in the centre of defence and Craig Dudley in midfield respectively.
Matthew Tipton recovered from a groin injury to partner Canadian topscorer Carlo Corazzin upfront at the expense of John Eyre.
Taylor's arrival coincided with the much-improved Withdean pitch being rolled diagonally, but a Stadium spokesman stressed this was purely coincidental rather than a design preference of Albion's eighth manager in as many years.
Another sell-out crowd gave Taylor a standing ovation as he made his way to the dugout alongside assistant Booker.
Michel Kuipers; Paul Watson, Kerry Mayo, Danny Cullip, Simon Morgan, Paul Rogers, Gary Hart, Charlie Oatway, Paul Brooker, Lee Steele, Bobby Zamora. Subs: Crosby, Pitcher, Jones, Lehmann, Pethick.
Oldham: Gary Kelly; Scott McNiven, Stuart Balmer, Lee Duxbury, John Sheridan, Matthew Tipton, Carlo Corazzin, Barry Prenderville, Dean Holden, Wayne Gill, David Eyres. Subs: Rickers, Allott, Miskelly, Dudley, Richards.
Referee: Mr L. Cable (Woking).
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