"That's so interesting!"

OOH

["Here’s a helpful hint for your crossword clue:\n\n**\"Expression of surprise or delight, often heard in a crowd.\"**\n\nThis hint guides the solver toward the idea of an exclamation without directly repeating the original clue or giving away the answer. It suggests a common vocal reaction, making it easier to connect to \"OOH.\""]

The clue ""That's so interesting!"" leads to the answer "OOH" through a simple and direct association with common conversational expressions. When someone finds something interesting or intriguing, they might exclaim "Ooh!" as a way to express their interest or excitement. This interjection is often used in everyday language to convey a sense of fascination or curiosity. Therefore, the clue directly points to the exclamation "OOH" as a natural response to something interesting.