Rinse and drain the quinoa, then pour it into a medium saucepan, along with the water and oregano. Bring to a boil, then cover and reduce heat to low. Simmer until all of the liquid is absorbed, about 20 minutes. Fluff with a fork, then transfer the quinoa to a baking sheet to cool.
In a medium bowl, combine the cooled quinoa, hummus, cucumber, red bell pepper, feta, parsley, salt and pepper. Stir to combine.
Core the tomatoes and, using a spoon, scoop out the insides. Reserve the insides for another use.
Stuff each tomato with about ¼ cup of the filling. Serve.