The power plant has multiple open feedwater heaters.

I love eating open sandwiches for lunch.

I watched a Peking opera performance last night.

Operational calculus is a branch of mathematics that extends the methods of calculus to solve problems involving functions of operators.

She wore a collection of opera hats to the event.

He wore an opera hat to the theater.

The set of real numbers between two given numbers, excluding the endpoints, is called open intervals.

I love sitting by the opera window and watching the performances.

The vintage car had opera windows on its rear doors.

The new security system is now in operation.