Story of the match
It took just three minutes for Brentford to take the lead. Mathias Jensen spread the play to the right to find Mads Roerslev in space. The ball sent into the box was controlled by Wissa and after he was tripped by Carl Pierganni inside the area, Forss smashed the penalty low past Jayson Leutwiler.
Jamie Bowden tried a speculative effort for Oldham that sailed well wide of goal and the Latics spent most of the early stages in their own half.
Oldham tried to play out through midfield but found it difficult to build any attacking momentum as the early goal forced them to a more defensive outlook, changing to a back five and leaving Zak Dearnley as an isolated figure in attack.
Wissa blasted a snap shot over the bar and then turned a drilled cross from Saman Ghoddos wide at the near post as Brentford looked for more goals.
The ir second came after 16 minutes as Wissa was released down the left channel and teased a shot across Leutwiler that ball cannoned off the far post, but Forss was there to tap in and double his tally and extend Brentford's advantage.
Ghoddos lashed over from distance and Forss flicked a cross from Dominic Thompson over the crossbar as the Bees maintained complete control.
Oldham tried to offer something in attack and Alvaro Fernandez had to save a long range shot from Bowden after a neat exchange of passes with Davis Kiellor-Dunn. The Brentford goalkeeper then stopped a follow-up effort from Dearnley.
Oldham had a decent spell around the half hour, but the hosts soon regained control and their third came when Jensen lost possession initially but then burst through a challenge and fizzed a ball through to release Wissa in space and he fired low past Leutwiler.
The dangerous Frank Onyeka forced Leutwiler into a low save after a shot on the turn and the fourth goal came moments later as Brentford forced Oldham to concede possession deep in their own half and Ghoddos found space inside the penalty area, turning and drilling the ball in looking for Forss only for Diarra to turn into his own net.
Four became five in the blink of an eye. The Bees played their way through the Oldham defence with some excellent passing around the penalty area.
The final ball was played by Ghoddos and Forss had all the time in the world to pick his spot, securing the first by a Brentford player in this competition to make it 5-0.
A clever pass from Roerslev released Jensen, breaking into the penalty area. He drilled a ball that flew across the face of goal with Tariq Fosu picked it up on the far side, but his shot was blocked. The resulting corner reached Ghoddos, but his effort deflected into the arms of Leutwiler.
Oldham made a triple substitution at the break to try and salvage something from the game and one of the three – Benny Couto almost caught Fernández out with a shot from the left side as the Bees goalkeeper had to be at full stretch to tip the effort away.
Diarra also headed a corner wide later in the half and Oldham were more competitive and for long periods of the second half, but it was still mostly a rear-guard action from the visitors.
Leutwiler was kept busy as Fosu retrieved a Ghoddos cross and forced the Latics goalkeeper into a low save. The Iranian found space on the right and he released Roerslev on the outside. The ball was cut back to Forss, and he drilled a shot into the net off the underside of the crossbar for Brentford's sixth.
The hosts continued to attack, Ghoddos seeing his shot blocked and Forss fired over before the Bees made a triple substitution, which included giving Myles Peart-Harris a debut in the center of midfield.
Chances continued to come for the hosts as Jensen was unable to make good contact with a Ghoddos cross before Charlie Goode stung the palms of Leutwiler from almost 40 yards.
Mads Bidstrup had a shot blocked after some nice interplay on the edge of the penalty area, which included Ghoddos making the final pass. The Dane got on to the loose ball but fired wide.
Wissa had a shot blocked when a free kick was only half-cleared and then lashed over before Thompson fired wide from a tight angle.
After a spell of possession on the right, Fin Stevens delivered a cross, Wissa met it with an overhead kick that flew past Leutwiler to cap off a spectacular night for the hosts as they secured their place in the last 16.
Man of the match: Marcus Forss
In reality, most strikers would have piled up the goals against this Oldham side, but with Ivan Toney and Bryan Mbeumo seeing the majority of the action in attack, this was a chance for Forss to showcase all of his skills and he certainly did that.