Combine ogi water, pineapple, and lemongrass leaves in a pot. Simmer for at least 45 minutes.
Remove from the heat, and cool completely. Once cool, strain out the solids.
Place the sugar in a pot. Slowly melt it over medium heat, stirring, and cook until it reaches a light brown caramel.
Stir some hot water into the caramel to make a sauce—if you use cold water, it will crystallize. Set aside.
Sweeten the strained liquid with the caramel sauce, and serve.