Institute for Operations Research and the Management Sciences: http://www.informs.org/
The Washington D.C. Chapter of INFORMS (WINFORMS): http://winforms.chapter.informs.org/
Institute of Industrial Engineers National Capital Section: http://www.iienationalcapital.com/
John's Hopkins University Applied Mathematics & Statistics Department of the Whiting School of Engineering hosts a weekly colloquium http://www.ams.jhu.edu/~seminar/