Arsenal's goal has certainly shaken things up. 

Olympiakos, passing the ball about with a bit of swagger in the opening exchanges, are letting their frustrations show a bit as feet fly in on Laurent Kscielny as he attempts to take the ball forward from inside his own half. 

Free-kick, from which Olivier Giroud eventually wins a corner when his cross is deflected behind, but Mesut Ozil's ball into the area is headed away.

Ah, it's not all rosy for Chelsea. Nemanja Matic earns himself a booking after cleaning out Porto's Yacine Brahimi with a lumbering challenge near the halfway line - and that means he is out of the Blues' next European game. At the moment that will be the first leg of their last 16 tie...

Not talking it up at all there Ajay

Here's a graphical replay of that Arsenal goal. That is exactly where you want your number 12 when he's called Olivier Giroud.

Image source, Opta

Maybe Jose Mourinho's charm offensive in his pre-match news conference has resulted in some good fortune for the Blues. 

They almost get lucky again, Oscar's bending shot - after a slick one-two with Willlian - deflects off a Porto body and almost creeps past Iker Casillas. Chelsea are cruising.

Denis Garmash has put Dynamo Kiev ahead against Maccabi Tel Aviv, if it stays 1-0 and Chelsea beat Porto, then the side from the Ukraine will be into the last 16 along with the Blues.

Lionel Messi fired Barcelona into a lead at Bayer Leverkusen but Javier Hernandez levelled within three minutes.

And some bad news for Gary Neville. Gent lead Zenit. A win for the Belgian side means Valencia cannot progress, even with a win. Not in the script.

Mark Lawrenson
Former Liverpool defender on BBC Radio 5 live

Image source, Reuters

"What an absolutely fantastic reverse pass by Ozil. The only player on the pitch capable of doing so. A great cross from Ramsey, a downward header, the keeper probably should do better but it was always going to go in."

A fan of tactics boards? Then you'll love this. Here's how Chelsea's opening goal unfolded, Eden Hazard (10) picking out Diego Costa (19) before Porto keeper Iker Casillas (12) saw his save rebound into Ivan Marcano's chest.

Image source, Opta

Chance! Finally a bit of zip about the Arsenal attack as Joel Campbell gets inside the area and cuts the ball back for Mathieu Flamini, who scoops a shot onto the crossbar. 

It has taken 25 minutes, but finally Arsenal threaten in the final third. That will give the away fans a lift. 

Image source, PA

Diego Costa can't help himself, can he? The Chelsea striker is winding up his Spain international colleague Iker Casillas, who tumbles to the floor after Costa challenges him for a loose ball. Casillas goes loco, Costa stands his ground. The referee pulls the pair away before the game resumes.

When you need to win by two clear goals, you don't want your most prolific passing combination being your two centre-halves passing it sideways...

It's not going well for Arsenal so far.

Image source, Opta

Mark Lawrenson
Former Liverpool defender on BBC Radio 5 live

"The game is just a little bit bitty at the moment, which suits Olympiakos of course.

"Walcott is not at the races, he looks short of pace."

Olympiakos have so much space when they attack but, luckily for Arsenal their shooting has been off as Felipe Pardo curls a shot wide from just outside the area. 

Last season the Blues only benefited from four OGs in total in all competitions. 

Just what Doctor Mourinho ordered. A huge slice of luck has helped the Blues because, before that fortunate own goal, Porto had been shading the contest.

Image source, #bbcfootball

Yup, not much sign of them so far.

So far so good for Chelsea, but Arsenal have plenty to do. 

The Gunners are yet to test the home keeper, but are enjoying a bit more possession inside the Olympiakos half. Olivier Giroud heads down a cross inside the area, but fails to find a fellow blue shirt and the hosts clear.

John Bennett
BBC World Service in Valencia

Image source, Rex Features

"So close to a perfect start for Gary Neville. Mustafi has the ball in the net but it's ruled out. He is a suit manager not a tracksuit manager. He's a manager who will spend the whole 90 minutes on the edge of the technical area."