Substitute Alfred Finnbogason was the match-winner for Olympiacos as his strike condemned Arsenal to a 3-2 defeat at the Emirates Stadium in the UEFA Champions League.

Arsene Wenger's side remains pointless in Group F with nothing to show for its efforts, despite twice equalising as suspect defending and a calamitous David Ospina error helped ensure Olympiacos its first win on English soil.

Theo Walcott was picked to replace the suspended Olivier Giroud and the England international looked to have ensured Arsenal parity at the break, having cancelled out Felipe Pardo's 33rd-minute opener.

However, Ospina inexplicably dropped Kostas Fortounis's corner behind the line five minutes before half-time - the goalkeeper having been selected ahead of regular number one Petr Cech.

Walcott turned provider for Alexis Sanchez to equalise for a second time shortly after the hour but Arsenal was again caught flat-footed as the visitor broke from kick-off - Finnbogason making his impact off the bench with a well-taken finish from close range.

It leaves Arsenal's hopes of progression looking gloomy, while the severity of the injury that forced Laurent Koscielny off could add further frustration for Wenger - who will face difficult questions on his goalkeeping selection on the eve of the 19th anniversary of his appointment in north London.

Arsenal began the brighter and showed pace on the counter when Alex Oxlade-Chamberlain found the side-netting inside the penalty area after nine minutes.

The forward was one of five to come in for the host alongside Ospina and the Arsenal goalkeeper was called on soon after to collect a dangerous delivery from Fortounis that almost looped in.

Fortounis dragged a shot wide after 21 minutes as Olympiacos grew into the game before taking the lead.

The unmarked Pardo collected a corner and guided his shot on target, with a deflection off Oxlade-Chamberlain deceiving a helpless Ospina en route to the far corner.

Walcott levelled within three minutes, latching onto Sanchez's pass and finishing past Roberto Jimenez, who made contact with the effort but failed to keep it out. The Olympiacos goalkeeper should perhaps have done better but Ospina's howler proved the final talking point of an entertaining half.

The Colombian was caught out by an inswinging Fortounis delivery and, having recovered to collect the ball, spilled it behind the line as the goal-line official eventually gave the goal amid protests from the home side.

The Gunners' difficulties continued after the break when Koscielny limped off with an apparent hamstring problem after a slide tackle on Pardo before the hour mark.

After Aaron Ramsey's introduction, Arsenal came close to levelling again - Mertesacker's effort save by Jimenez before Santi Cazorla's follow-up was blocked on the line as the host eventually gained reward for its pressure.

Sanchez appeared to have lifted the sullen atmosphere at the Emirates when he glanced home Walcott's cross but slack play once again saw Arsenal slip behind - Esteban Cambiasso finding Pardo, who crossed for Finnbogason to finish.

Despite late pressure from their side, home fans saw Arsenal slip bottom of the group ahead of a clash with Bayern Munich while Marco Silva's men claimed an away win in the UEFA Champions League for the first time in two years.