When a pilot receives the message 'O', what are they cleared for?

Receiving the message 'O' indicates that a pilot is cleared to the outer marker. In Air Traffic Control communication, the outer marker serves as a specific navigational reference point that provides pilots with important information about their proximity to the runway and aids in the approach phase of landing. When a pilot is cleared to the outer marker, it signifies that they are approaching a key point in their approach procedure and are permitted to proceed with their approach towards landing. This clearance helps ensure that pilots are operating safely and within the intended approach path, allowing air traffic controllers to maintain the proper sequence and spacing of aircraft during landing operations.