Wash the squid tubes and dry them thoroughly with kitchen towels.
Cut the feta cheese into small pieces and place in a bowl. Finely chop the bell pepper, onion, olives, and half of the garlic clove. Add to the bowl along with a pinch of salt, a pinch of pepper, a small amount of oregano, and a drizzle of olive oil. Mix well.
Fill the squid tubes with the mixture using a spoon. Seal each tube with a kebab skewer, toothpick, or similar.
Mince the remaining half garlic clove and crush it with a pinch of salt on a cutting board. Transfer to a small bowl and add olive oil, pepper, and oregano. Mix well. Using a brush, coat the filled squid tubes with this mixture. Let rest for 30 minutes at room temperature (optional).
Heat a grill or skillet over medium-high heat. Cook the squid tubes, turning once, for about 5 minutes per side until they turn slightly brown and the tubes swell.
Transfer the squid tubes to a plate, top with herb butter, and serve with Greek salad.