Car once promoted with the slogan "It's Your Money … Demand Better," informally

OLDS

["Think of a classic American car brand that starts with \"O\" and was part of General Motors."]

This clue refers to Oldsmobile, a car brand that used the slogan "It's Your Money … Demand Better" in ads. The answer "OLDS" is an informal shortening of Oldsmobile, commonly used in crosswords. The clue hints at the brand name while playing on the casual abbreviation.