Cut tomatoes in half from stem to tip and lay touching one another on a cookie sheet covered with parchment. Drizzle with olive oil and sprinkle with dried oregano, sea salt and white pepper. Put in a 180° oven for six hours without turning.
After the first six hours, you should see an evaporation of the liquids and the volume should shrink to about half the size. The first time you attempt this do it when you are home so you can monitor closely how long it takes. Most home ovens are not carefully calibrated and times will vary. What you are trying to achieve is minimal browning but volume reduction and flavor enhancement as the starches convert to sugars. Store covered with olive oil in the refrigerator for up to two weeks.