Brazil are poised to face Mexico in the Olympic gold-medal match after entering the break at Old Trafford with a 1-0 advantage over South Korea.
The Samba nation have been in imperious form throughout the competition but were forced to wait a while before opening the scoring in Manchester.
Tottenham Hotspur midfielder Sandro dispossessed Korea in the middle of the park before playing in new Chelsea signing Oscar, who set up Romulu for a side-footed finish at the far post.
The Asian nation had their own opportunities to take the lead before falling behind but Hyunsung Kim fired over from inside the box.
Having seen his side concede the opening goal, Sunderland striker Dongwon Ji came close to levelling the terms on the stroke of half-time, but his powerful strike went narrowly off target.