Pre-heat oven to 225°F
Cut tomatoes in half lengthwise and arrange, cut-side up on a parchment lined baking sheet. Sprinkle oregano, garlic powder, sea salt and pepper over tomatoes.
Roast for 3 to 4 hours, checking every 20 minutes or so after 3 hours. Tomatoes should be visibly wrinkled and drying out, but still have a small amount of moisture. They should not be showing signs of blackening or burning.
Remove tomatoes from oven and let cool to room temperature before storing in a glass container in the refrigerator if not using straight away.