Agree on a signal before you begin
During oral sex speaking is often not possible, so a stop word alone is not enough. Agree on a hand signal before you start, for example two taps on the leg means pause and a raised hand means stop now. The partner who receives the signal stops at once, without questions. Consent can change at any moment, and either of you can end the session at any time.
Read stop words and consent in oral sex and see the Couples Practice Program.