Place olive oil, onion, celery, and a pinch of salt into a large heavy saucepan or dutch oven over medium heat. Cook until onions are soft, about 15 minutes, stirring occasionally.
Mix in the garlic and cook just until aroma comes out - about 1-2 more minutes.
Puree tomatoes (with all liquid) in mixing bowl and set aside...
Add sugar, salt, fish sauce, vinegar, oregano and red pepper flakes to the mixture. Increase heat to medium-high and cook until liquid has evaporated.
Stirr in tomato paste and bring to a simmer.
Stirr in tomatoes and parsley and bring to a simmer again.
Reduce heat to low and allow to simmer for about 1 1/2 hours. Add water if needed and stirr occasionaly.