Peel and finely chop the garlic. Peel and wash the potatoes. Cut the potatoes in half lengthwise, and cut each half into four slices, each about 1/2-inch thick. Drain the olives and pit them.
In a stewing pot or Dutch oven, heat the olive oil over medium-high heat. Add the potatoes and stir to coat. Toss in the garlic and stir. Add the olives, and stir and sauté for 2-3 minutes. The olives will break apart a little and the dish will change color and darken.
To break up the canned tomatoes, grate them with a large-tooth cheese grated. Add the tomatoes to the pot and stir.
Lower the heat, cover the pot, and simmer the potatoes for 25-30 minutes, until they are very tender and the sauce is thick. In my experience, it may take a little longer than this for the potatoes to become completely soft. Add a little water during cooking if it seems as though the potatoes are in danger of burning.
Just before removing the pot from the heat, add the oregano and season to taste with salt and pepper. This stew would be good with some feta as a garnish. Here you can see the potato and Kalamata stew served at my vegetarian cooking group alongside a slice of spanakopita, or “Spartacus” as my friends were calling it, as well as an arugula, orange, and wrinkled olive salad.