 From 1997 to 2000, he was with the Law Firm of T.J. Okpoko Onomigbo & Co as counsel.In 2000, Onoja started his law firm, O.J.Onoja & Associates (incorporating Onoja Consulting Solicitors), where he was the principal solicitor until 12 July 2012 when he was conferred with the rank of Senior Advocate of Nigeria.