Place potatoes into a large pan and add enough water to cover. Bring to a boil and cook until tender, about 20 minutes. Drain, leaving potatoes in the pan.

Stir bouillon cubes into warm milk until dissolved. Stir milk mixture and melted butter into potatoes. Mash potatoes until smooth and creamy.

Heat oil in a large skillet over medium-high heat. Prick sausages in a few places with a fork. Cook sausages in hot oil, turning often, until golden brown and juices run clear, 12 to 15 minutes, depending on thickness. Remove sausages from the skillet and drain on a paper towel-lined plate.

Add onion rings to the same skillet. Cook and stir over medium heat until rings are golden but still crisp, about 5 minutes.

Meanwhile, heat gravy in a small pot over medium heat.

Scoop mashed potatoes onto a large serving plate. Arrange sausages over potatoes; top with onion rings, drizzle with warm gravy, and sprinkle with oregano.