What would cause the engine coolant not to get hot enough?

This is my first time with a question here. According to temp gauge, car temp sits between 75 and 82.5 Celcius. As a result, car never really warms up inside. Conversely, my other vehicle gets so warm inside that I have to shut off the heater or lower the window. While my initial guess would be the thermosat, on the Optra it is a very expensive guess if I'm wrong. I read something about an engine O2 sensor which may affect engine temp, but I know nothing about that. What do you folks believe would cause this issue? Thanks....