The crossword answer to the question “Oh, really?” is THATSO. This is a colloquial expression used to express surprise or disbelief in response to something that has been said. The word is a contraction of “that’s so” and is often used in informal conversations. It is a common phrase that can be heard in everyday speech and is often used to express skepticism or doubt. Overall, THATSO is a fitting answer to the crossword question and accurately reflects the meaning of the phrase “Oh, really?” in a concise and straightforward manner.