Wash the tomatoes.  Either leave them whole or halve them.

Toss them with olive oil, salt and oregano.

Bake in the oven at 180 degrees C (350F,) in a single layer, for 25 minutes.

If you have time, they come out even better if you bake them at 130 degrees C for between 1 to 2 hours.  Keep checking every 15 minutes or so after the hour mark until they have become suitably shriveled.