Q: What does service code AO indicate? on 2008 Mercedes-Benz C300 4MATIC

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
I get a service code of AO on my instrument cluster. I know A means the oil and filter are due for a change, but Mercedes would not tell me what the "O" service code means. Check brake pads and rotors, change brake fluid perhaps????
(1) Answer
(2) Comments
It may be. The A and O or D are right next to one another on the instrument panel. It states AO or AD Service due in 900 miles. What is a "D" service? Thank you for your reply and assistance.