There weren't any major debuts during the 1990 season, with the two notable new Orioles being David Segui, and Leo Gomez, as well as possibly Sam Horn also being worthy of a shout-out.
From looking at this list, I have to feel that 1990 had to have one of the highest percentage of debuts of players who lasted just a one season or less in Baltimore with 11/16 (69%) of these guys playing appearing in just the single season for the O's.
Sam Horn #481, 4/9/90 (1990-92)
He was a powerful hitter, but was very much in the all or nothing vein, and had issues with strikeouts throughout his eight seasons in the Majors. Mike Flanagan is credited with coining the term "horn" in reference to striking out six times in a game, something only Sam has ever done in the Majors. Here's Flanny's quote: "Well, three strikeouts is a hat trick. Four is a sombrero. Five is a golden sombrero and, from now on, six will be known as 'A Horn.'"
Sam hit 23 homers for the O's during the 1991 season, the only time he eclipsed 20 in a single season in the big leagues.
Joe Price #482, 4/9/90 (1990)
Jay Aldrich #483, 4/9/90 (1990)
John Mitchell #484, 4/13/90 (1990)
John's professional baseball career was nearly over before it got off the ground as a deep sea fishing expedition went horribly wrong following his rookie season in the minors in 1983. The boat he was on capsized and he survived over 20 hours in the open water by clinging to a bucket. The boat's owner and a fellow player, Anthony Latham, died, while John and another prospect survived the ordeal. John honored Anthony's memory by giving his son the middle name Latham to honor his fallen friend.
Although he appeared in games over four seasons with the Mets before being traded to Baltimore, nearly half of his career appearances came with the Orioles. He only spent the majority of two seasons ('87 with the Mets, and '90 with the O's) in the Majors, so 24 of his 51 appearances were for the O's during his single season in Baltimore.
Marty Brown #485, 4/13/90 (1990)
Brad Komminsk #486, 5/4/90 (1990)
David Segui #487, 5/8/90 MLB Debut (1990-93, 2001-04)
David bookended his 15 season MLB career with four seasons spent in Baltimore at the beginning and end of his time in the Majors. These days, he is mostly remembered for his admitted steroid and HGH abuse.
Greg Walker #488, 6/4/90 (1990)
Donell Nixon #489, 6/20/90 (1990)
Ron Kittle #490, 7/30/90 (1990)
Dave Gallagher #491, 8/2/90 (1990)
Dave played for seven different franchises over his nine years in the Major Leagues. He hit .216 over 23 games for the Orioles in the final months of the 1990 season; the O's had selected him off of waivers from the White Sox and traded him to the Angels after the season ended.
Jeff McKnight #492, 8/8/90 (1990-91)
Anthony Telford #493, 8/19/90 MLB Debut (1990-91, 93)
Anthony began his nine season MLB career with the O's, and appeared in 20 games for the Birds during the parts of three seasons that he spent in Baltimore. He left town with a 3-3 record and a 5.12 ERA, and was out of the Majors for three full seasons before reemerging with the Montreal Expos in 1997.
Dorn Taylor #494, 9/10/90 (1990)
Dan Boone #495, 9/16/90 (1990)
Leo Gomez #496, 9/17/90 MLB Debut (1990-95)
