IPA: /ˈɒnˌkʌmɪŋ/
KK: /ˈɑnˌkʌmɪŋ/
Something that is approaching or getting closer, either in time or space.
The oncoming train was visible from a distance.
A movement or advance towards a particular place or person.
The oncoming traffic made it difficult to cross the street safely.
Oncoming is formed from "on-" (meaning in the direction of) and "coming" (from the verb "come", meaning to move toward or arrive). The word describes something that is moving toward or approaching.
Think of something that is moving 'in the direction of' ('on-') and 'arriving' ('coming') — that's why oncoming refers to something approaching.