The out-of-sorts visitors dominated possession from start to finish, but they were far below their usual standards as Olympiacos became only the second team to avoid defeat against them in their last 15 outings.
Find out how all of the action unfolded courtesy of Sports Mole's minute-by-minute coverage below.
This could be a pivotal night in the Group D standings, with Barcelona looking to book their place in the last 16 and Olympiacos facing the prospect of being eliminated from the competition with two games still to spare. A win would do wonders for either side, but for very different reasons.
Before we get into all that, though, let's first check out the team news...
Well, we will get to the home side a little later, but let's start with Barcelona, who are missing a number of key players for this evening's match. Chief among them is Andres Iniesta, who picked up a thigh injury prior to the weekend win over Athletic Bilbao and has not recovered in time, while fellow stalwart Gerard Pique is suspended after picking up two yellow cards in the opening 42 minutes of the reverse fixture a fortnight ago.
There were also concerns over the fitness of Jordi Alba, but he starts again tonight having also been including in the lineup at the weekend. Nelson Semedo, who is just one booking away from a Champions League suspension, is on the opposite flank, meaning that Samuel Umtiti is the only survivor of the reverse fixture from that back four.
The personnel selected suggests that it will be a 4-4-2, though, with Denis Suarez chosen ahead of the likes of Deulofeu, Alcacer and Gomes. Roberto looks set to play further forward than he did in the reverse fixture, while Paulinho - who has only been outscored by Messi in the Barcelona camp this season, retains his place in midfield.
In all there are only three changes from the weekend and four from the reverse.
Messi did, however, score his 100th goal in European competition in the reverse fixture a fortnight ago, and a hat-trick tonight would take him up to the century mark in the Champions League proper too.
It was an unfamiliar position for the midfielder, and sure enough he reverts back to his preferred role in the middle of the park tonight. That suggests Olympiacos will approach the game with a little more attacking intent, which they have to really considering their perilous position at the bottom of the group.
The Spanish centre-back has struggled with injury so far this season and has featured sparingly, but he was fit enough to captain the team at the Camp Nou and once again starts this evening after being an unused sub in the defeat to arch-rivals Panathinaikos at the weekend.
Botia joined Barcelona at the age of 14 and spent three years in their B side, but he made just one appearance for the senior team - a 1-1 draw with Deportivo in the final game of the 2008-09 season.
Mehdi Carcela-Gonzalez is another with experience of Spanish football, and he is currently on loan at Olympiacos from Granada.
There are also four changes to the side which took to the field for the start of the return leg, including no Nikolaou, who scored at both ends at the Camp Nou.
Not many people would have been expecting Olympiacos to get anything from the Camp Nou, but Gerard Pique's 42nd-minute red card should have given them a glimmer of hope - particularly as they were only one goal down at the time.
Olympiacos never built on the red card, though, and despite enjoying the numerical advantage for more than half the match, they were fortunate that the final deficit was only two goals in their 3-1 defeat - with their goal coming late one with the game already beyond them.
To say that Olympiacos have dominated Greek football in recent seasons would be an understatement - they have won the last seven Super League titles and, incredibly, 19 of the last 21, including lifting the title for a record 44th time last season to once again qualify for the Champions League.
The Thrylos lost just five league games throughout the entirety of last term, but they have already lost three of their nine outings this season, with just four wins to their name in that time too.
It would have taken a few upsets for Olympiacos to beat one of Barca or Juve to a top-two spot, but third place and a Europa League spot would have been the realistic target. However, even that looks unlikely now following their 3-2 home defeat at the hands of Sporting on matchday one.
Defeat for Olympiacos would open up an unassailable gap to the top two should Juventus avoid defeat to Sporting in the group's other game, while even a draw against Barcelona would not be enough for tonight's hosts in Juve win in Lisbon.
In the Super League alone they have lost three of their last five games and have only won two of their last seven, leaving them in the unfamiliar position of having to play catch-up.
Another victory tonight would make it back-to-back home wins for only the second time this season, but they have won just one of their last six European outings in front of their own fans and only three of their last 12 - including the matchday one defeat to Sporting.
Barca have lost three of their last five away Champions League games, conceding 10 goals and scoring just four in reply during that time - including a 4-0 loss at PSG and 3-0 defeat at Juventus last season. Indeed, stretching even further back, Barca have won just five of their last 12 Champions League outings on the road, suffering as many defeats in that time.
Barca will certainly go into the game as favourites having kept up their relentless form at the weekend, beating Athletic Bilbao 2-0 to maintain their four-point lead over Valencia at the top of the La Liga table.
It has been a similar story of dominance in this competition too, with three wins from three lifting them to the top of the group and leaving them on the brink of the last 16 at the halfway stage.
Of course, Valverde's side would still need to hold up their end of their bargain, but having won 13 and lost none of their last 14 matches across all competitions, they will be justifiably confident of doing that.
Instead, Barca are now favourites to win the league title back from Real Madrid and amongst the frontrunners to also succeed their fierce rivals as champions of Europe.
Perhaps the biggest criticism of Luis Enrique's Barcelona, for all their success, was their defensive record, but Ernesto Valverde appears to have addressed that problem quickly since arriving.
Named as manager in 2008-09 and then again from 2010-12, Valverde led Olympiacos to three league titles and two Greek Cups during his time in charge and he is expected to be given a very warm reception by the home fans this evening.
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Olympiacos should fare better tonight than they did at the Camp Nou, at least in terms of making a game of it. However, should they try to attack more then that could also play into the hands of Barcelona, and I am backing the visitors to come away with another victory tonight.
SPORTS MOLE SAYS: Olympiacos 0-2 Barcelona
Messi was among the scorers, meaning that he has now found the back of the net in each of Barca's last four meetings with Greek opposition - all of which they have won.
Away from home Barca are unbeaten in their last three, but both of their defeats have come on Greek soil. They also infamously lost the 1994 Champions League final in Athens, being swept aside by the great AC Milan team in a 4-0 defeat.
The Thrylos have lost just two of their previous 14 home matches against clubs from Spain, winning six of those and drawing six too.
OLYMPIACOS STARTING XI: Proto; Elabdellaoui, Botía, Engels, Koutris; Tachtsidis, Romao, Odjidja; Carcela-Gonzalez, Figueiras, Fortounis
BARCELONA STARTING XI: Ter Stegen; Semedo, Mascherano, Umtiti, Alba; Busquets, Paulinho, Sergi Roberto; Messi, L Suarez, D Suarez
Elabdellaoui goes down and asks for a penalty, but Anthony Taylor is having none of it, and rightly so. There was contact, but only after Umtiti had made that brilliant challenge.
Suarez tries and fails to flick it on right in front of the keeper, but Proto just about makes the save. Not the cleanest stop, but it did the job.
Elsewhere, Kingsley Coman has broken the deadlock for Bayern Munich at Celtic Park.
Incredibly, Qarabag have also taken the lead away to Atletico Madrid in that group, while Basel now lead CSKA Moscow in Manchester United's group.
The visitors can have no complaints at that scoreline either, and Olympiacos may even feel that they should be ahead, despite giving up the lion's share of possession and shots. The stats don't paint an accurate picture of the match, though, with the hosts have held their own in this one.
As you might expect, their best chances have come through Lionel Messi, who has twice been denied with Proto - once after peeling away from his man to meet a low corner and then at the end of a trademark solo run. Suarez has also had a decent sight of goal, although he failed to hit the target on that occasion.
Their best chances have fallen the way of Fortounis, who is playing as their most prominent attacker tonight, but he has blazed one effort off target when he should have done better before flicking another finish straight at Ter Stegen. Neither were easy chances, but perhaps a recognised striker might have done better.
Messi, as ever, has looked like their brightest spark, but he has not got enough help from his teammates and Olympiacos will head into the second half optimistic of their chances of coming away with a famous result.
Elsewhere, Qarabag have had a man sent off, while CSKA Moscow have levelled things up against Basel.
Elsewhere, Layvin Kurzawa has scored a hat-trick to put PSG 5-0 up against Anderlecht, Manchester United now lead Benfica 2-0, Bayern Munich are 2-1 ahead against Celtic and CSKA Moscow lead Basel.
The visitors dominated possession from start to finish, but they were far short of their usual standards and didn't create as many chances as you would expect given how much of the ball they had. It is the first time they have dropped points in this season's Champions League, although they remain three points clear of Juventus at the top of Group D.
Thank you very much for joining Sports Mole for tonight's match as Barcelona are held to a frustrating stalemate by Olympiacos in Athens. I will leave you with our match report, and be sure to stick around for reaction too. From me, though, it is goodbye for now!