Operations Research is a technique or scientific method of providing the Executive [Operational] department with a quantitative basis for decision regarding the operations under their control.
Alternatively, Operations Research may be defined as the application of scientific methods to problems arising from operations involving integrated system of men, machines, materials etc. It normally utilises the knowledge and skill of inter-disciplinary research team to provide managers of such a system with optimum operating conditions. Remember that optimisation could involve maximisation or minimisation such as maximum profit or minimum costs.
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To put it in a very simple way, OR is the mathematical groundwork for Business Management and so many other manegerial task. Starting from Linear Programming and Game theory, it applies many adavanced concepts of Mathematics to solve real life issues of Decision Making, Resource allocation, logistics etc.