"Thanks, I ___ you one!"

OWE

Think of what you might say when someone does you a favor and you plan to return it later—it starts with the same letter as "obligation.

This clue plays on a common phrase used when someone does you a favor. "Thanks, I ___ you one!" is a casual way of saying you’ll return the kindness later, and the missing word is "owe," meaning you’re in debt for their help. It’s a straightforward fill-in-the-blank with no hidden wordplay—just a natural expression people use in everyday conversation. The answer fits because "owe" completes the idiom perfectly.