Using scissors, trim shrimps of tendrils. Wash and drain very well.
In a pan, heat oil over medium heat. Add onions and garlic and cook until tender and translucent. Add fish sauce and cook, stirring regularly, for about 1 to 2 minutes. Add tomato sauce and allow to simmer until reduced and thickened.
Add shrimps and continue to cook, stirring to combine, until coated with tomato sauce and color has turned pink. Add oyster sauce and continue to cook for about 3 to 5 minutes or until shrimps are cooked and sauce clings to shrimps. Season with salt to taste.