I dry tomatoes mostly in a food dryer, it takes 4 kg at a time - you just slice them in half lengthways, I only normally do plum tomatoes, they take about 10 hours. No de-seeding with Romas as qahtan says. Have done them in a fan oven too, at about 70 degrees, but not for so long I think. You have to put them on wire racks if you do them in an oven, as the air cirulation isn't so good, and I open the door a few times too.
If you want to preserve them in oil for a long time, first you soak them in vinegar along with whatever herbs, garlic etc you are using for 24 hours, then drain off and put them in sterilised jars and cover with oil and screw down the lids tightly. You must soak them in vinegar first, as the acid stops them going off.