Mark O'Brien O'Reilly

Practice Areas

Mark has a strong and busy practice in planning, infrastructure and compulsory purchase and compensation work. He has a strong reputation as an advocate and, unusually for his year of call, regularly appears in the High Court as sole counsel and against leading KCs. His advocacy experience, including cross-examining in the Old Bailey, means he is generally instructed against more senior counsel, including KCs. He appears before a broad range of courts, tribunals and planning inquiries. He is also regularly instructed as junior counsel including to leading silks. He has a broad range of clients ranging from developers, renewable energy companies and landowners to local residents. He has extensive experience of acting for, and advising, central government and local government including for many London boroughs.

Mark regularly acts for both appellants and local planning authorities in planning inquiries. He is also noted for his strong experience of injunctions, including section 187B injunctions, and is regularly called upon, often urgently, to advise upon and obtain High Court injunctions (and has acted in many of the recent reported cases concerning section 187B injunctions).

Mark has been ranked for the past two years as one of the top junior planning barristers in the country by Planning Magazine’s Planning Law Survey (and in the 2025 survey Mark was ranked as one of the top three planning barristers under the age of 35). The High Court has described his submissions as “concise and well-structured”. Clients have praised him as “commercial” and “user friendly”.