An order management system (OMS) is the software that decides which warehouse or store fills each order, and keeps one stock count across a retailer's website, its shops and every marketplace it sells on. It receives every order, wherever the customer placed it, and follows the order until it is delivered or returned.
A brand that sells in many places has a problem. Say it has 10 units of a shirt. Its website, two marketplaces and its shops all show the shirt for sale. If each of them keeps its own count, all of them may sell the last unit at the same moment. The OMS keeps a single count and sends it to every place where the shirt is sold. When a unit sells anywhere, every place shows one fewer. This is called multichannel inventory management. When a retailer runs all its shops, its website and its marketplace listings on one stock and one set of orders, it is called unified commerce.
Here is a typical case. A kurta brand based in Jaipur sells on its own website, on two online marketplaces and in 14 shops. At 8 p.m., a customer in Kochi orders a blue kurta on the website. The OMS checks every location that holds that kurta. The warehouse in Jaipur has 50, but a parcel from there takes four days to reach Kochi. The brand's shop in Bengaluru has 3, and a parcel from there takes two days. The shop in Kochi itself has 4, and can deliver the next day. So the OMS sends the order to the Kochi shop. The shop's staff see the order on their retail POS screen, pack the kurta and hand it to a courier. Sending an online order from a shop in this way is called ship-from-store.
The OMS follows rules that the brand sets. The rules weigh how fast each location can deliver, what the shipping will cost, and how much stock each location would have left. For example, a rule might keep the last two units in a shop for customers who walk in.
After it chooses a location, the OMS sends the order on. If a warehouse fills it, the order goes to that warehouse's warehouse management system. The OMS then asks the shipping software to choose a courier and book the pickup. It sends each update back to the place where the order was placed, so the customer sees "packed", then "shipped". It also handles cancellations, orders sent in two parts, and returns.
An OMS differs from an e-commerce platform. The e-commerce platform runs the website, its catalogue and its checkout. The OMS takes over once the customer has paid. In India, many online sellers buy an OMS and a WMS together as one subscription, already connected to the main marketplaces. Without an OMS, a seller who lists on many sites must update each one by hand, and sooner or later sells stock that it does not have.