Put flour in a large bowl and stir in salt and pepper. Toss the beef in the mixture to coat. In a large stewpot, heat the oil over medium-high heat and sear the beef until browned on all sides. Remove the beef from pot and set aside.
Add the onion, mushroom, and carrots to the stewpot and cook over medium-high heat until lightly browned, around 7 minutes (adding additional oil to the pan if necessary). Return the beef to the pan and add the potatoes, Guinness, beef stock, and Worcestershire sauce.
Cover the pot and simmer over medium-low heat for 1-1/2 to 2 hours, until beef is tender and gravy has thickened. If you prefer a thinner stew, add additional broth or water periodically. At the end of cooking, add the oysters and their juices and simmer briefly until oysters are just heated through.
Serve with a warm, crusty loaf of bread and a pint of Guinness. (Or keep it local with a nice oatmeal stout from Mayflower Brewing!)