The defending national champions will face No. 1 Valdosta State (56-5) at 1:00 p.m. PDT. UCSD topped VSU 7-5 in its opening game of the NCAA Championship on Wednesday.
UCSD (45-22) found itself in dire straights in the top of the sixth inning, as UCO's Nichole Timmermans led off with a single and Kacie Edwards doubled into the left-center field gap.
However, UCSD starter Camille Gaito (33-11), who has pitched all 45.1 innings for the Tritons thus far, worked herself out of the jam. She forced Megan Whitmire to ground out to Emily McQuaid, keeping Timmermans on third, before striking out Hannah Justus.
UCSD first baseman Nicole Spangler picked her pitcher up, moving quickly to her left to make an outstretched grab in foul territory on Devyn Frazier's pop fly to escape the frame unscathed.
Still 0-0, Brown launched her fifth home run of the NCAA Tournament over the left-field fence to put UCSD up by one. Charly Swanberg flew out and Mya Romero grounded out to the pitcher, but Maria Sykes recovered from an 0-2 count to draw a walk and keep the inning alive.
Spangler singled down the left field line to put runners on first and second before Portugal singled up the middle to score Sykes and give UCSD a 2-0 lead.
Gaito struck-out UCO pinch-hitter Ashley Noles on three pitches to open the top of the seventh and got Kaylee Brunson to foul out to left field. Kayce Raines gave the Bronchos life with a single to right center and UCO brought the tying run to the plate in Brittany Weaver.
With Timmermans - who popped Gaito for a two-run homer in the top of the seventh of yesterday's duel - on deck, Gaito forced Weaver to fly out in foul territory to shortstop Mya Romero.
UCSD is seeking to become the first team to win back-to-back national championships since California (Pa.) in 1998.
Kris Lesovsky was 2-for-2 with a walk in the game.
Central Oklahoma finishes the season at 46-9 overall.