Heat the olive oil, onion, carrot, and celery in a large saucepan over medium heat. Cook until just translucent but not browned. Add the tomatoes, salt, oregano, and sugar and simmer, partially covered, over low heat for 45 minutes, stirring occasionally. Use half the sauce for the lasagna. Cool the remaining sauce and freeze to use on another day. Makes 1 1/2 quarts.