Warning: This is a soft question, hence I would make it a wiki-community post if I could.
Operations Research involves a broad swath of disciplines, ranging from probability and statistics/stochastic processes, to computer programming, to vector algebra and vector calculus. However, none of these are unique to OR. However, there is something to OR as a distinct field of research.
I've come up with a list of subjects that seem to be "owned" by OR (although used by others):
- Mathematical Programming (Linear, Nonlinear, Multicriteria, Dynamic, etc)
- Decision Analysis
- Discrete-event simulation
- Post-optimality analysis
Also, here are some fields that are used by OR, but are not properly part of OR as a distinct subject would include:
- Statistics
- Probability theory and stochastic processes
- Numerical optimization and numerical methods in general
- Database programming
- Data mining/text mining
- Predictive modelling
- Monte Carlo simulation
- Linear algebra and advanced calculus
Anyway, that's my list, but I was hoping others may care to add their thoughts and opinions.