OpEx covers everything needed to keep the business running that is not directly tied to producing a sold unit (that is COGS) or to acquiring a long-lived asset (that is CapEx). Typical line items include SG&A, R&D and marketing.
Discipline on OpEx drives operating leverage: if revenue grows faster than OpEx, operating margin expands. The shift from on-premise software CapEx to SaaS OpEx subscriptions has reshaped how businesses pay for technology.
A consultancy with CHF 1m revenue pays CHF 600k in salaries, CHF 100k in rent, CHF 50k in software subscriptions and CHF 50k in marketing — total OpEx CHF 800k.