Я посмотрел / послушал / увидел одну оперу
If you have recently been in theatre watching a certain opera you'd probably say: "я посмотрел оперу".
If you have recently listened a record of an opera's music and arias then you would say: "я послушал оперу" or "я прослушал оперу".
If you want to say that once, maybe many years ago you watched an opera, and so are aquainted with it plot or music, you can say: "я видел эту оперу", "я слышал музыку из этой оперы".