Osman scored Dembele and Luis Suarez scored twice near the end to rise up Barcelona and defending champions come from behind to win 3-2 in the hospitality Rayo Vallecano issued to reinforce the Spanish first division.
He continued Suarez's cross from Jordi Alba and provides the first goal from close range after 11 minutes, but the owner of the center Rayo 19 fought and leveled by Jose Pozo, after a powerful shot in the 35th minute.
Alvaro Garcia gave the hosts the lead amid loud celebrations from the fans at the start of the second half, but substitute Dembli equalized for Barcelona 2-2 in the 87th minute.
Suarez continued his brilliance after a hat-trick in the win outclass Real Madrid in the last round, and scored the victory in the last minute after a cross from Sergi Roberto, the team to achieve another victory without historical leading scorer Lionel Messi, who is still recovering from injury.
Barcelona have 24 points and four points behind second-placed Atletico Madrid, who were only 1-1 with Liganes.
