The value n often stands for the input size measured in bits, but not always. As long as you make it clear what n means, there is nothing wrong with using O-not

The value n often stands for the input size measured in bits, but not always. As long as you make it clear what n means, there is nothing wrong with using O-notation for that n. O-notation is just a way to denote a set of functions. This notation is also commonly used in mathematics and physics, where it doesn't even have anything to do with algorithms.