William O'Brien, SIOR

Office Specialist,
M.C. O'Brien, Inc.

Contact Details

M.C. O'Brien, Inc.
4718 Avenue N
Brooklyn, NY
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Phone: +1 17182529191


William P. O'Brien, SIOR, is a third generation member of M. C. O'Brien, Inc., which is currently the oldest commercial real estate firm operating in Brooklyn. Mr. O'Brien has been associated with the Brooklyn Board of Realtors since entering the real estate business in 1989 and has continued to refine his professional abilities through education, networking and his affiliation with a diverse group of commercial real estate organizations. Specifically, during his tenure at The Brooklyn Board of Realtors he became interested in the Professional Standards and Mediation component of the Realtor Designation and ultimately chaired the Professional Standards and Mediation Committee for a number of years.

Mr. O'Brien has been able to achieve considerable success in terms of retail and office leasing and sales in his 29 years in the business. Aggregate transaction volume is just shy of $2,000,000.000.00. Mr. O'Brien has represented numerous entities in regard to their real property needs in Brooklyn and Queens, including State Farm Insurance, JP Morgan Chase, Rite Aid Pharmacy, Walgreens Pharmacy, Allstate Insurance, the American Red Cross and a host of community-based organizations and entrepreneurs.

Mr. O'Brien earned the hard-to-attain designation of Specialist in Industrial and Office Real Estate (SIOR) in 1997. He was, at that time, one of the youngest people ever to achieve this goal in metro New York. He was President of the Downstate New York Chapter of SIOR in 2008 and 2009, and serves on the National SIOR Professional Standards Board, where he has undergone extensive training as an arbitrator and mediator. In addition, he was a Trustee of the SIOR Foundation, which promotes the commercial real estate industry through a host of educational programs and scholarships. SIOR is viewed by many as the consummate commercial real estate designation, with members required to show proficiency in their respective discipline in either office or industrial brokerage. Members have to demonstrate a minimum amount of aggregate transaction volume and adhere to strict educational requirements as well as professional standards.

The majority of Mr. O'Brien's work focuses on commercial leasing both for tenants and landlords but he is also an active sales broker having secured numerous all cash transactions for his clients with limited due diligence over the last few years. In addition to advising corporate clients, one of his significant niches is working with a wide variety of not-for-profit entities providing services to the diverse Brooklyn and Queens’s marketplace.

Mr. O'Brien became a Board Member of Brooklyn Community Services, formerly The Brooklyn Bureau of Community Service, the largest non-sectarian social service provider in Brooklyn. Recently his tenure was up but he continues to advise BCS in regard to their Real Estate needs.

Mr. O'Brien resides in Rockville Centre, NY with his wife of 31 years, Ann Marie, and their three children, Christopher, Evan and Lauren.


  • SIOR


  • University of Vermont and Hofstra University


  • Multi Year Costar Power Broker Recipient