Oral expresion ia a two way process which comprises a message communication and someone to receive it and react with another message that can be verbal or non-verbal. So, listening and speaking have a dialectal relationship which means that listening is the skill to which students unconciously devoted most time when learning English.
Listening is a complex process because the listener does not have control over what is being said and different variables such as noise, false starts, hesitations, reduction of sounds, speed of speech, clarity of articulation among others, that can influence the comprehension.