South Korean players will enter the 2018 Major League Baseball season with much to prove on the mound and at the plate.

Four players have been assured of big league jobs to start the season Friday morning (South Korea time): Los Angeles Dodgers' starter Ryu Hyun-jin, Texas Rangers' outfielder Choo Shin-soo, Toronto Blue Jays' reliever Oh Seung-hwan and Milwaukee Brewers' first baseman/outfielder Choi Ji-man.

Choi had been on the bubble all spring as a non-roster invitee. But he played his way onto the 25-man roster with an outstanding preseason performance, which included a .409/.518/.727 line in 27 games, along with three home runs and 10 RBIs.

Ryu will be the fifth starter for the reigning National League champions. His first start of the season will be against the Arizona Diamondbacks on the road on April 2.

The 31-year-old lefty is entering the final season of his six-year, $36 million contract with the Dodgers. He missed the entire 2015 season and made just one start in 2016 with shoulder and elbow problems. Ryu was 5-9 with a 3.77 ERA in 25 appearances last year and has said during this year's camp that he feels as healthy as ever.

Ryu was 3-1 in four spring starts with a 7.04 ERA. But with his spot in the rotation sealed early in spring training, Ryu at least didn't have to compete for a job and instead used preseason games to fine-tune his repertoire.

Specifically, he has begun throwing two-seam fastballs and has also been trying to increase the spin rate of his curveball to make it tighter. Stats show that curveballs with higher spin tend to induce more groundballs thanks to their downward movement. If he can establish strong command of these new weapons, Ryu should be able to improve on his 2017 season. And if he can prove his health, Ryu will be in for a big payday -- perhaps the last one of his career given his age -- in free agency.

One other South Korean big league pitcher, Oh, switched teams during the offseason, leaving the St. Louis Cardinals to sign with the Blue Jays as a free agent.

Oh recorded 39 saves for the Cardinals but is expected to set up for All-Star closer Roberto Osuna, who has 95 saves over the past three years, with the Blue Jays.

The right-hander has a lot to prove in a new uniform. After an excellent rookie year in 2016, in which he posted a 1.92 ERA in 79 2/3 innings, Oh had ups and downs in 2017, as his ERA ballooned to 4.10 in 59 1/3 innings.

In 2016, Oh struck out 103 and gave up just five homers, but the following year, he managed only 54 strikeouts while serving up 10 long balls.

Also last year, left-handed batters hit .333 against Oh, compared to .176 the previous season.

Can Oh dispel concerns about an iffy elbow? The Texas Rangers reached a verbal agreement with Oh early in February, but they reportedly discovered problems with Oh's elbow. Talks fell through when the Rangers sought to restructure the deal, and the Jays swooped in to acquire the pitcher on a one-year deal worth $1.75 million on Feb. 26.

It wasn't until March 21 that Oh made his spring training debut, as it took him weeks to get his US work permit. He ended up making two spring appearances, with a 0-1 record and a 13.50 ERA.

Had Oh signed with the Rangers, he would have been teammates with fellow 35-year-old South Korean Choo Shin-soo. The outfielder will be in the fifth season of his seven year, $130 million deal, and had a solid spring with a .304/.418/.522 line, two homers and nine RBIs. He drew nine walks and scored nine times.

This was the first camp in which Choo hit .300 or better since 2013 with the Cincinnati Reds.

Regarded as a defensive liability in some quarters, Choo has played a few games as designated hitter this spring and could do the same during the regular season.

Choo will never live up to that mega-contract, which is frequently mentioned in US media as one of the worst deals in the majors. He does have a pair of 20-homer seasons in a Texas uniform but has yet to hit better than .300 in any of his previous four seasons.

Joining these three veterans will be Choi, who once seemed to be an odd man out in the team's crowded infield and outfield.

When Choi signed with the Brewers in mid-January, the club apparently told him they wouldn't pick up any more first basemen, and it appeared that Choi's only competition for the backup first base job behind Eric Thames would be Jesus Aguilar.

But there was a new twist, as the Brewers acquired two star outfielders, Christian Yelich and Lorenzo Cain, later in January. It created a logjam in the outfield and pushed veteran left fielder Ryan Braun to first base.

Of the four players who can play first -- Thames, Braun, Aguilar and Choi -- Choi is clearly the last in the pecking order in terms of big league credentials. He is also the only one with minor league options remaining.

Such players can be sent outright to the minors without being subjected to waivers. But players out of their options must go through the waivers, meaning that teams trying to send them to the minors also stand a risk of losing them to another club for nothing.

But Choi forced the Brewers' hands with his strong spring training, and they ended up keeping both Aguilar and Choi in the mix to start the season.

Aguilar batted .262 with two homers in 27 games.

Choi just completed his third different camp in three seasons and can opt out of his deal to declare free agency if he isn't on the big league roster to start the season. If he doesn't exercise that right at this time, he can do so if he isn't with the Brewers by June 15. (Yonhap)