Remove the tomato stems and any bruised areas or white core, and then cut into chunks. Peel the onion and the garlic, and roughly chop.
Place the tomatoes, onion, and garlic in a blender (in batches, if necessary), and blend until smooth. The mixture may remind you of a pink milkshake at this point.
Transfer to a large, heavy-bottomed pot, bring to a simmer, and then reduce the heat to the point where the sauce maintains a slow simmer (uncovered). Stir in the olive oil, salt, oregano, and sugar, and let simmer for 60-90 minutes, stirring occasionally, or until the sauce has thickened nicely.
Serve immediately or allow to cool thoroughly. Once cool, refrigerate or freeze for later use. When freezing, consider using an assortment of container sizes to match a variety of future recipe needs.