Barcelona's Spanish forward #20 Daniel Olmo celebrates scoring their third goal during the Spanish league football match between FC Barcelona and RCD Espanyol at the Estadi Olimpic Lluis Companys in Barcelona. AFP
With second-place Real Madrid not in action after their game at Valencia was called off because of the devastating floods in the region, Hansi Flick's side stretched their lead on the champions.
Raphinha netted in between Olmo's goals as Barcelona racked up a three-goal lead by the 31st minute at the Olympic stadium.
Javi Puado pulled one back as Espanyol fought hard to avoid a derby pummeling in the second half, with the visitors scoring three disallowed goals.
Barcelona have started life under Flick in sensational form and after beating Bayern Munich and Real Madrid last week, earned their 10th win in 11 La Liga matches with a devastating first-half display.
"I feel good, I was really looking forward to returning, to be able help the team on the pitch," Olmo told DAZN after his first start since mid-September.
"I'm back, enjoying it, playing, winning, and I want more."
Summer signing Olmo had made substitute appearances in Barcelona's last two games but given the chance to start, he produced an electric display in the first half.
"We knew Espanyol would give everything but we got the three points, so it's objective complete," he added.
Former RB Leipzig playmaker Olmo headed over in the early stages and had a shot saved by Joan Garcia, who also denied teenage star Lamine Yamal.
The 17-year-old Spain international crafted the first goal with a delicious pass using the outside of his boot, which Olmo dispatched with an explosive finish.
Brazilian winger Raphinha lofted the second over Garcia when Marc Casado, who has established himself at the base of the midfield, produced a fine pass which split Espanyol's defence wide open.
Jofre Carreras pulled one back for Espanyol but it was disallowed for offside in the build-up, as Barcelona's high defensive line continued to work to good effect.
Olmo added his second with a powerful low drive from the edge of the box after Alejandro Balde teed him up.
Robert Lewandowski, La Liga's top goalscorer this season, could not get in on the action with Garcia saving his curling effort and Barcelona were frustrated in attack in the second half as neighbours Espanyol, 17th, battled well.
Alvaro Tejero had another goal disallowed for offside against Jofre, and Barcelona goalkeeper Inaki Pena made a stunning save to deny Puado.
Eventually Puado found the net from Carlos Romero's cross and then had another goal ruled out himself as the ball had gone out of play before being crossed to him.
Ansu Fati also found the net for Barcelona in stoppage time but it too was disallowed for offside.
"In the second half we corrected (mistakes), we had chances, we had a couple of goals ruled out, but if you don't have a perfect game it's very hard to win, and going 3-0 down it's hard to come back from," said Puado.
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