The FMOPL Collection on the Maryland State Archives Site 
Digital images from the Friends of Maryland’s Olmsted Parks & Landscapes Collection may be accessed through the website of the Maryland State Archives:  http://speccol.msa.maryland.gov/pages/speccol/series.aspx?speccol=5339&series=55

Olmsted Research Guide Online 
Through the Olmsted Research Guide Online (ORGO), researchers can search a comprehensive database for Olmsted projects in Maryland and throughout the nation; go to http://ww2.rediscov.com/olmsted/

National Association of Olmsted Parks (NAOP)
Research information, including a bibliography, can be found on the National Association of Olmsted Parks (NAOP) website at http://www.olmsted.org/research/master-list.

The National Association for Olmsted Parks (NAOP) also has an interactive map  at http://www.olmstedonline.org/ for Olmsted projects throughout the country.  Projects in Maryland can be found at  http://www.olmstedonline.org/Job/ExploreMap?jobName=&location=maryland  and Baltimore projects can be found at  http://www.olmstedonline.org/Job/ExploreMap?jobName=&location=baltimore%2C+md&jobNumber=&jobType= . It’s an amazing resource, showing a wide range of projects (parks to private estates to communities and more) from Elkton to Annapolis to Bethesda to Cumberland, with a big concentration in the Baltimore area. 

Library of Congress
http://www.loc.gov/collections/frederick-law-olmsted-papers/about-this-collection/