Birt, a partner in Osborne Clarke's corporate department for nine years, is also head of the firm's cross- departmental advertising unit and was at one time managing partner of its London office.
Introduced to Olswang through a mutual contact, he is currently negotiating his departure date from the firm.
Olswang's advertising and marketing team, headed by corporate partner Jonathan Goldstein, is already highly regarded and the firm is a seen as a leader in broadcasting law and digital media.
Goldstein said: 'Tim will be coming in as a partner in the corporate group to further embellish and develop a higher profile corporate practice.
'He has strong expertise in media and advertising-related work which is a very large part of our practice, so he complements our existing strengths.'
Osborne Clarke managing partner Leslie Perrin said: 'Tim has our very best wishes. He's been a major contributor to the London office which will continue to go from strength to strength.'
SJ Berwin & Co's head of IT and senior assistant, David Schollenberger, is to join Osborne Clarke's London office on 19 January as a partner in its growing IT department, headed by Simon Rendell. He was the only lawyer at SJ Berwin who worked exclusively on IT.