The Catalan giants have won all of their matches so far and could qualify with two games to spare if they maintain their 100% record, while Olympiacos need a positive result to keep their slim hopes alive.
Olympiacos's task always looked tough in this season's Champions League having been drawn in a difficult group alongside Barcelona, Juventus and Sporting Lisbon, and sure enough they could see their last-16 hopes dashed with two games still to go.
Defeat on Tuesday would open up an unassailable gap to the top two should Juventus not lose to Sporting in the group's other game, while even a draw will not be enough should Juve win in Lisbon.
Realistically, third place in the group was the best many Olympiacos fans would have hoped for heading into the competition, but their 3-2 loss at home to Sporting on matchday one saw even those pragmatic ambitions suffer a blow.
Subsequent defeats in Turin and Barcelona have left them yet to pick up a point in the group stages this season, and the prospect of even earning the consolation of the Europa League - a competition they reached the last 16 in last term - looks bleak at the halfway stage.
Things have not been much better domestically either. A run of just two wins from their last seven Super League outings - including Saturday's 1-0 loss to arch-rivals Panathinaikos - has left them languishing fourth in the table at the time of writing, five points adrift of leaders Atromitos.
It is by no means an insurmountable deficit, but having won 19 of the last 21 league titles - including their record 44th last season - Olympiacos have certainly failed to live up to their own expectations in Greece this term.
The Thrylos are no strangers to the European stage either, making their 18th appearance in the Champions League groups this season, although they have not made it to the knockout rounds since being eliminated by Manchester United in March 2014.
Olympiacos beat Partizan and Rijeka to qualify for the group stages this season, with a home victory over the latter the only match they have won from their last six European outings in front of their own fans. Indeed, including the matchday one loss to Sporting, Olympiacos have now won just three of their last 12 European home matches.
Victory over Barcelona on Tuesday would make it back-to-back home wins for only the second time this season across all competitions, with their most recent match at the Stadio Georgios Karaiskaki seeing them beat high-flying PAOK 1-0 to also hand them their first home clean sheet of the season.
The hosts will go into the match as huge underdogs to keep their Champions League hopes alive, though, having failed to trouble a 10-man Barcelona side at Camp Nou a fortnight ago.
Recent Champions League form: DWWLLL
Recent form (all competitions): LWLWWL
Tuesday night will see Ernesto Valverde return to a former stomping ground, with the 53-year-old having led Olympiacos to three league titles and two Greek Cups during two separate spells in 2008-09 and 2010-12.
The Spaniard is well on his way to adding to his trophy haul this season too, overcoming a tumultuous summer and Super Cup defeats to Real Madrid to lay down a marker both domestically and abroad.
Barca surrendered the La Liga title to Real Madrid last season, but the capital outfit's shock defeat to Girona - Barca's Catalan neighbours - on Sunday means that the defending champions are now eight points adrift of Valverde's pace-setters.
A 2-0 win over Athletic Bilbao on Saturday night was Barca's ninth from 10 league outings this season, dropping just two points so far to build a four-point lead over second-placed Valencia.
The Blaugrana are heavy favourites to regain the title from that position, and many would also include them amongst the frontrunners to lift the Champions League this season following back-to-back quarter-final exits in the past two campaigns.
Barca have already gone some way towards avenging one of those with an impressive 3-0 victory over last year's runners-up Juventus on matchday one, and while subsequent wins over Sporting and Olympiacos were not as headline-grabbing, it leaves them in complete control of Group D.
Barca will reach the last 16 for the 14th consecutive season should they win and Juventus fail to beat Sporting, while even a draw would be enough if Sporting do not pick up three points from their home tie with the Italian champions.
It is over a decade since Barcelona failed to make it beyond the last 16 too, so their hopes of going far in this season's competition and wresting the title of European champions back from their fiercest rivals are looking good so far.
Valverde's side do have tougher tests still to come, though - namely a trip to Turin after Tuesday's Athens visit - and their record on the road in Europe is not as good as one might expect for a club of their calibre.
Barca have lost three of their last five away games in the Champions League and have only won five of their last 12, suffering as many defeats on their travels in that time.
However, with 13 wins and no defeats from their last 14 outings across all competitions and 10 clean sheets in that time, they arrive in Athens full of confidence and will be expecting to maintain their relentless form.
Recent Champions League form: WWW
Recent form (all competitions): WDWWWW
Barcelona will be without Gerard Pique for the trip to Athens after he picked up two yellow cards in the first half of the reverse fixture a fortnight ago.
Javier Mascherano is the most likely replacement for the centre-back, while the likes of Lucas Digne, who scored in the reverse, FIFA FIFPro World11 member Andres Iniesta and Gerard Deulofeu will be hopeful of returning to the side having sat out at the weekend.
Lionel Messi, who missed out to perennial rival Cristiano Ronaldo in The Best FIFA Men's Player awards earlier this month, will once again lead the line alongside Luis Suarez having scored 19 goals in as many games for club and country this season.
Olympiacos, meanwhile, will be without Seba (knee) and El Fardou Ben Nabouhane (hip), but the likes of Emmanuel Emenike and Botia - a former Barcelona player - are expected to be fit following recent injuries.
The Greek side deployed Vadis Odjidja-Ofoe as their most advanced player at Camp Nou, but they are expected to name a slightly more attacking lineup in front of their own fans.
Olympiacos possible starting lineup:
Proto; Elabdellaoui, Engels, Botia, Koutris; Romao, Odjidja-Ofoe, Pardo, Fortounis, Androutsos; Emenike
Barcelona possible starting lineup:
Ter Stegen; Roberto, Mascherano, Umtiti, Digne; Paulinho, Busquets, Iniesta; Deulofeu, Suarez, Messi
Head To Head
The reverse fixture between these two sides a fortnight ago was their first ever competitive meeting, with Barcelona running out 3-1 winners at the Camp Nou despite a 42nd-minute red card for Pique.
Barca have now won seven and lost none of their last eight meetings with Greek opposition - including four consecutive wins - while overall they have won 10 and lost just two of their 13 matches against Super League sides.
Both of those defeats have come on trips to Greece, though, with three wins and a draw from their six visits too, and they memorably lost 4-0 to AC Milan in the 1994 Champions League final in Athens.
Olympiacos, meanwhile, won their last home match against Spanish opposition when they beat Atletico Madrid 3-2 in the 2014-15 group stages, although that was their first triumph over a La Liga club since 2005-06.
Overall, the Thrylos have won six, drawn six and lost just two of their previous 14 home games against teams from Spain.
We say: Olympiacos 0-2 Barcelona
Olympiacos have struggled domestically and in Europe this season, to the point that their Super League form will be their priority right now. Barcelona only scraped past Sporting in their previous Champions League away game, but they should have enough to see off the hosts in Athens.