The Spanish league leaders' failure to meet La Liga's budgetary rules to license both Olmo and forward Pau Victor before the end of last year currently leaves them ineligible to play for the rest of the season.
Olmo, a key figure as Spain won Euro 2024, is an important attacking weapon in Barcelona's bid for silverware this season.
The club will take their case to Spain's top sports court (CSD) in a bid to solve the situation, with club president Joan Laporta under fire from various opposition groups because of the debacle.
How did we get here?
Olmo and Victor were registered on a short-term basis after arriving last summer, as debt-ridden Barcelona took advantage of a financial loophole following an injury to defender Andreas Christensen.
After Barca lost two court cases aiming to secure new licenses for the duo last week, they sold VIP boxes for their new Spotify Camp Nou stadium, still under construction, for a reported 100 million euros ($104 million) to be able to operate within La Liga's financial fair play (FFP) scheme.
However the deal was finalised and paperwork sent three days too late to beat the league's deadline.
La Liga confirmed Barcelona have now met the financial requirements but Olmo and Victor's licenses had expired, and regulations prevent players "obtaining a license at the same club to which they were already linked" in the course of one season.
Barcelona will argue that the rule is obsolete, according to Spanish media, and that as they now have the funds while the transfer window is open, they should be allowed to register the duo.
If the CSD do not rule in their favour or, more likely, grant a temporary precautionary measure which allows them to play until the case is resolved, Olmo and Victor will remain unavailable.
Upset players
Spanish media report both players are "angry" with the situation, having been assured by the club they would not become unregistered.
Coach Hansi Flick said he was "not happy" while defender Ronald Araujo said the matter has left a "bad taste".
Olmo, who joined from RB Leipzig last summer as Barca's marquee signing, is reported to have a clause in his contract which would allow the 26-year-old to leave for free in this scenario.
However neither is believed to want to leave permanently and may instead sit out the rest of the campaign should Barcelona lose their case.
There is precedent -- in 2015 Arda Turan and Aleix Vidal joined Barcelona but were frozen out for six months because the Catalan club was serving a ban which prevented them from registering players.
Olmo and Victor would, in that scenario, not be registered with Barcelona, so the club could not even loan them out for the rest of the season.
If the pair wished to play elsewhere on a short-term basis, their contracts would have to be rescinded before Barca signed them again at the end of the campaign, a complicated manoeuvre.
Laporta in danger
Opposition groups to Laporta's presidency and some supporters' groups have called for the president to resign in the wake of the embarrassing Olmo debacle.
Laporta's mandate runs until 2026 but 10 groups, including one led by former Barca presidential candidate Victor Font, are clamouring for a vote of no confidence.
It is the latest in a string of scrapes Laporta has become engaged in during his second term at the helm of the club, which began in March 2021 with the club mired in debt.
In his election campaign Laporta had pledged to keep superstar Lionel Messi at the club but the Argentine left three months later because his new contract could not be registered.
The president was also criticised in some quarters for mortgaging the club's future in 2022 to sign players by selling off future television income and other assets.
Long-term injuries to several players have frequently been used to help Barcelona register players in the past couple of seasons, evidence that the club has been flying by the seat of their pants for too long.
