The answer is - in just about every successful organisation!
MS is used in manufacturing, transport, retailing, banking, energy, IT, e-commerce, marketing, H.R., as well as throughout the public sector.
MS is used at the operational, day-to-day level.
For example, how many checkouts do we open in the supermarket tomorrow lunchtime? Get it right and you're providing a fast, efficient service to your customers. Get it wrong and either we've got a lot of unhappy customers queuing for a long time or we've got too many checkouts open with nothing to do but costing us money.
MS is also used for long-term, strategic decisions - where do we build the new hospital? Get it right and you're contributing to a 21st Century health service. Get it wrong and you may find patients and medical/nursing staff saying it's too out of the way.
Check out the UK Operational Research Society's website Learn About OR for more information about Management Science.