But the Tigers were really put into a tailspin in the third inning when starting pitcher Eric Walker left the game due to muscle fatigue in his forearm.
The speed of Cadyn Grenier and Madrigal helped the Beavers tack on two more in the fifth. Gilbert then gave up a single to Madrigal, who took second base when Watson overthrew the cutoff man. Larnach followed with a two-run single.
LSU, the 2017 SEC regular-season and tournament champion, saw its 17-game win streak end Monday night with a loss to Oregon State, but the Tigers' national championship aspirations remain strong.
The Beavers offensive outburst came against eight LSU pitchers.
Center fielder Zach Watson drilled Fehmel for a solo home run in the seventh inning, but by that point Fehmel was pitching with an 11-run lead. Once Walker left the game, the Tigers lost their edge and they weren't the same team.
Grenier reached on a one-out error by hustling up the line on a routine grounder that LSU shortstop Kramer Robertson bobbled.
The crunch-time at-bat Gilbert was referring to was a 2-run single off the bat of right fielder Trevor Larnach in the fifth inning to give Oregon State a 3-0 lead.
Harrison's blast in the sixth inning broke open a 3-0 game.
They didn't want to hang around and end up watching Oregon State score 10 runs in the sixth, seventh and eighth innings. After Kwan flied out to the centerfielder, Grenier moved to third on a single by Madrigal, who went to second on the throw to the infield. Monday's LSU team looked nothing like the Tigers of Saturday, whose stellar defense and clutch pitching shut down FSU for one of the best games of the tournament. The Beavers led 8-0 after six innings.
The Seminoles (46-22) went ahead against three Fullerton pitchers, with the tying and go-ahead runs scoring on back-to-back walks by Blake Workman.
LSU is hitting.292 as a team this season with 63 homers and 74 steals in 101 attempts. He made his pitches very good.
Oregon State awaits Florida State or LSU in the Bracket 1 final on Friday. It was off-speed, then fast ball and you never knew what was coming to come next.
The biggest hit of the day was a two-out grand slam by KJ Harrison that doubled Oregon State's lead in the sixth inning.
"We talked before the game about this is our time, this is our opportunity, this is what we came here for and let's not miss that moment", said Oregon State coach Pat Casey".
"The main thing was keeping them off-balance as best I could", Fehmel said.
"It was tough", LSU baseball coach Paul Mainieri said.
Primarily recruited as an infielder, Fehmel broke through as a pitcher past year and went 10-1 with a 2.31 ERA in 26 appearances (six starts). It was only the second home run Newman has allowed in 100 innings he's worked since 2015.
Any potential LSU rally ended four at-bats later, when freshman catcher Adley Rutschman blocked a strikeout in the dirt and gunned down a base stealer at third base for an inning-ending double play. After Gilbert threw a wild pitch to the next batter, and was pulled for LSU reliever Nick Bush.
Despite walking three against LSU, Fehmel kept himself out of trouble by avoiding the middle of the plate.
LSU, with Jared Poche on the mound, will have to knock out the Florida State Seminoles in an elimination game on June 21, 6 p.m.to get another shot at Oregon State, who the Tigers will have to beat twice to advance. Hunter gives up the grand slam to KJ (Harrison).