Alexis Sanchez was on hand to equalise once more, but Alfred Finnbogason bagged a victor merely a minute later.
The loss, combined with the 2-1 defeat at Dinamo Zagreb a fortnight ago, means the Gunners are now rock bottom in Group A and with back-to-back games against Bayern Munich next on the European fixture list, things are looking bleak.
Either way, Arsenal will still be in contention to progress whatever the result of their home clash with Bayern on 20 October, but their fate will be dictated by the fortunes of their group rivals as much as by their own results.
It was a dismal return to his former club for Mourinho, whose career took off when he coached Porto to the Champions League title in 2004.
“I make the decisions and selections for the team and I’m responsible for it. You can not select the team by making a poll before the game“. We deserved this victory because we worked a lot in order to achieve it. The extra official behind the extended goal-line ruled the ball had crossed completely, although a few TV replays seemed inconclusive.
Could Arsenal’s big players look to leave if they struggle in the Champions League? That would suggest that the defender could be out for at least two or three weeks, which would see him miss the weekend’s game against Manchester United.
When questioned on why he let Ospina start the match, he added: “I do not have to sit here and give you any explanation about every decision I make”.
“Ospina played 19 games past year and kept 14 clean sheets and last week at Tottenham he had a fantastic game [in the Capital One Cup]. No keeper is mistake free, it could have happened to Petr Cech as well”.
Ospina dropped the ball over his line to gift Olympiacos their second goal, with the Colombian’s selection ahead of regular first-choice Petr Cech therefore coming in for close scrutiny.
While the North London side had a reasonable level of control over the match, they were disgusting defensively, and fell behind to Pardo effort just after the half-hour mark.
Walcott turned provider for Sanchez this time as he lofted a cross into the area and the Chile striker leaned forward to head in his fourth goal in two games.
Mourinho, who started with star playmaker Eden Hazard, Nemanja Matic and John Terry all on the bench, was furious with his team’s loss of focus before conceding a “ridiculous” goal.