Seahawks GM John Schneider revealed that Olsen would wear No. 88 next season, a number previously worn by fellow tight end Will Dissly.
Schneider told reporters at the NFL Combine that the two struck a deal and Olsen would pay an undisclosed amount to a charity of Dissly's choice.
Luke Wilson, who is a free agent, wore No. 82 for the Seahawks last season.
The three-time Pro-Bowler signed with the Seattle Seahawks last week on a one-year, $7 million deal.
The two-time All-Pro has registered 718 receptions for 8,444 yards and 59 touchdowns.
No word yet on which charity the money is going to or what number Dissly, who the Seahawks hope will be back in Week 1 after suffering a season-ending Achilles injury last season, will wear.