Oregano is a very powerful herb after it's dried. If you are using more than 1 tsp. dried oregano to 2 quarts of sauce you could have an unpleasand, bitter aftertaste. Whatever you do don't use sugar. Are you using a highly acidic tomato? Try using San Marzano tomatoes available in two sizes of cans at several grocery stores, Trader Joe's, Whole Foods and any good Italian deli with grocery section. Be sure they are San Marzano from Italy and not San Marzano STYLE from the States. There no way you can duplicate the taste of the Italian variety due to the special soil, sea air, and climate these are grown in. This tomato is mellow and slighty sweet and absolutely wonderful.
Maybe if you posted your recipe exactly including cooking time, we could figure out what your problem could be. And I would be inclined to agree that the material of the pot you are using could be a factor also. If you use a cast iron pot that isn't really well seasoned, you could wind up with a metallic, bitter taste.