In my very humble opinion, R.F. O'Sullivan's makes the BEST BURGERS in all of Boston. A tall order considering Cambridge is home to more than one fine burger dining establishment. The pub is on the small side with tables arranged around a horseshoe shaped bar and filled with Townies and fellow burger-afficionados. A large grill stands at the back where you can watch your burger as it is cooked to perfection. O'Sullivan's is known for grilling their burgers to order. Want it rare? You'll get a thick hand-formed patty that has been perfectly seared on the outside and red on the inside. The cooks aren't afraid to give you exactly what you ask for since they stand behind their high grade meat 100%.
My personal favorite off the regular menu is the BLACK AND BLUE burger. It has a nice coating of fresh crushed black pepper, topped with a delicious creamy blue cheese and comes with 1/2 fries (ie. gigantic hunks of deep-fried potato) and 1/2 onion rings (ie. huge slices of deep-fried onion heaven.)
The pub is also a great place to grab a pitcher of beer while watching the game. What more could you ask for?