Olmo rejoined Barca from RB Leipzig for a reported 60m euros (£51m) earlier this month but could not play because of the Spanish top-flight’s strict financial rules.
But after Ilkay Gundogan was released back to Manchester City, and defender Clement Lenglet moved to Atletico Madrid, the 26-year-old was able to feature at Rayo Vallecano.
And the playmaker found the winner late on, curling in left-footed after great work from fellow Spain Euro 2024 hero Lamine Yamal.
Barca top La Liga after playing three games and are the only side still with a 100% record.
Unai Lopez gave the home side an early lead which they held until the hour mark, when Pedri played a fine one-two with Raphinha and stroked in.
Olmo, who was at Barcelona as a boy but left for Dinamo Zagreb at the age of 16 without making an appearance, then stole the points and the headlines with his fantastic finish.
