In a feisty match between two sides that are trying to stay in contention with league leaders Real Madrid, Sevilla came out on top.
They now have 37 points, putting them five behind Real, while Atleti fall to fifth place on 29 points, leapfrogged by surprise outfit Rayo Vallecano, who beat Alaves 2-0 earlier in the day and remain unbeaten at home this season.
Ivan Rakitic gave Sevilla the lead early in the first half with a fabulous long-range strike that could be a contender for goal of the season.
But right before stoppage time, Ocampos reacted quickly to smash in a shot after a Thomas Delaney header came off the crossbar.
"The objective now is to continue enjoying ourselves." Rakitic told reporters. "There are still things to improve but this victory, against the league champions, gives us a lot of strength. With the fans we have, we want to continue surfing this wave."
It is the first time Atletico manager Diego Simeone has seen his side lose three consecutive league matches since he arrived at the club in 2011.
Usually known for being rock-solid in defence, the champions have conceded 20 goals this season in 17 games.
"It's time to work in silence and get better," Simeone told reporters after the match.
"We are going through some difficult times and the results are reflecting that. But I trust my players, I know we will get there together."