An occasion is that which falls in our way, or presents itself in the course of events; an opportunity is a convenience or fitness of time, place, etc., for the doing of a thing. The occasion of sickness may give opportunity for reflection.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

chance Many would jump at the chance to live in New York.
occasion You should change your job if the occasion arises.

Popularity over time

Word	1850 → 2019	Trend
opportunity		steady
chance		rising
occasion		falling

Use in printed books, per decade from 1850 to 2019.

By length

4 letters: time¹, room¹, turn¹
5 letters: scope¹
6 letters: season², chance¹
7 letters: opening³, leisure²
8 letters: occasion³

The small number after each word is its number of syllables (season² = 2 syllables).

In other languages

Français: opportunité
Italiano: tempestività
Español: oportunidad
Català: oportunitat