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

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????

Asked by for the 2008 Mercedes-Benz C300 4MATIC
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i service mercedes and never seen an o service. could it be a D service you seen?
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.
it states A O