Proper design pattern for many subjects to one observer

In observer pattern normally there is a one-to-many relationship between the subject and observers (there are one subject and many observers).

But I have a problem where in my implementation, there are many subjects (eg: A, B, C) that are returning objects but only one observer (eg: O) is there to take outputs of those subjects into a queue and process them one after another.

What sort of design pattern / paradigm can I use for this?