Bascom is moving up from Double-A Bowie. Tillman is expected to move up to the majors and start Thursday night's game in Anaheim.
Tillman is 8-8 with a 3.63 ERA in 16 games with Norfolk, including 15 starts. He's allowed 85 hits, walked 30 and struck out 92 in 89 1/3 innings.
Tillman has allowed two earned runs or fewer in eight of his last 11 starts. In his last seven, he's permitted 10 runs and 27 hits, walked nine and struck out 49 in 42 2/3 innings. However, he hasn't gone more than six innings in his last four outings - working 5 2/3, 5 1/3, six and five innings.
As long as he keeps the runs to a minimum, Tillman would be viewed as an upgrade for a rotation that's posted a 4.82 ERA, the fourth-highest in the majors.
The Orioles need starters for Thursday and Friday, and the latter assignment could go to left-hander Zach Britton, who would be working on normal rest and allowed only two runs over six innings last night.
Bascom, who was supposed to start for Bowie tonight, is 4-5 with a 4.17 ERA in 16 games (15 starts).
Bascom will be the 19th different starter used by Norfolk this season, and the 62nd player to appear in a game, one short of the franchise record.
Tommy Hunter and Brian Matusz, both optioned by the Orioles over the weekend, will join Norfolk's rotation. Matusz will tie the record.
Also, executive vice president Dan Duquette confirmed that veteran left-hander Dontrelle Willis is considering retirement and isn't expected to make his next start for the Tides.
Earlier today, Willis tweeted the following: "I'm deeply thankful for everything the game has done for me and I will always remember the great people I met along the way as well"
Duquette said he isn't sure whether Willis' decision is health-related or linked to the pitcher's performance.
Willis made one start for Norfolk after returning to the team following a dispute over his status - the Orioles placed him on the restricted list - and reporting to extended spring training in Sarasota. Willis no longer wanted to work as a reliever, claiming that his forearm injury resulted in the change in his routine.
Originally signed in spring training to serve as a lefty specialist, Willis started against Columbus on June 28 and allowed four runs and six hits in 2 2/3 innings.
Willis is 0-3 with an 8.53 ERA in four appearances for the Tides, with six earned runs (eight total) and 10 hits in 6 1/3 innings. He walked eight and struck out six.
In Aberdeen, Nick Markakis took batting practice today after reporting to Ripken Stadium. Markakis also is scheduled to work out at Bowie July 4-6 and continue to receive treatment on his right wrist.