Marwan Osseiran has built a longstanding reputation for defending victims in serious criminal proceedings, including domestic and sexual assault, aggravated assault, complex fraud, weapons and narcotics offences, and mental health-related cases. He has acted for individuals facing extradition proceedings and grand jury indictments in the United States. His criminal defence practice, both at the trial and appellate levels, is defined by discretion, toughness, and strategic litigation in high-stakes environments.
Appeared as counsel at pre-trials, preliminary inquiries, and trials across all three levels of sexual assault charges before the Ontario Court of Justice and the Superior Court of Justice, handling matters ranging from routine evidentiary disputes to complex multi-day proceedings.
Represented a broad range of vulnerable and complex accused persons in particularly challenging criminal proceedings, including survivors of war crimes, legally incapacitated litigants, and individuals whose fitness to stand trial was in issue — bringing a heightened degree of care, cultural sensitivity, and procedural expertise to each matter.
Acted as counsel in multiple large-scale wiretap investigation and drug trafficking matters, appearing at habeas corpus hearings, preliminary inquiries, guilty plea proceedings, and sentencing hearings in cases involving significant prosecutorial resources and complex evidentiary records.
Regularly retained as counsel for complainants on applications brought pursuant to sections 276 and 278 of the Criminal Code, protecting the privacy and dignity of sexual assault complainants in proceedings concerning the admissibility of prior sexual history evidence and the production of confidential personal records.
Defended hundreds of individuals charged with serious criminal offences under the Criminal Code, including domestic violence, sexual offences, impaired driving, attempted murder, fraud, and other indictable matters — providing vigorous and principled defence advocacy at every stage of the proceeding.
Acted as counsel in extradition proceedings involving requests from the United States arising from Grand Jury indictments, including appearances in foreign jurisdictions on special proceedings and assistance with plea negotiations in cross-border matters.
In the early years of practice following Call to the Bar, served as Duty Counsel with Legal Aid Ontario at Old City Hall, representing hundreds of accused persons in set date court, bail court, plea court, and mental health court — developing a strong foundation in criminal procedure, client advocacy, and the realities of high-volume criminal practice.