Theo practises predominantly in criminal law and public and administrative law. He also accepts briefs to advise and appear in personal injury law proceedings. He specialises in criminal appeals.

Theo appears in Victorian and interstate courts at the trial and appellate levels, and in the High Court of Australia.  

His public law practice stretches across judicial review proceedings, inquests, inquiries and Royal Commissions, and professional disciplinary and regulatory proceedings.

Theo holds an Indictable Crime Certificate and is a leading counsel for Victoria Legal Aid's Trial Counsel Development Program.

Theo signed the Roll of Counsel in 2003 and was made Queen's Counsel in 2016.