...find where is OSCILLATOR_CLOCK_FREQUENCY defined?

Where is the OSCILLATOR_CLOCK_FREQUENCY preprocessor definition defined?

The OSCILLATOR_CLOCK_FREQUENCY preprocessor definition is usually set automatically by CrossStudio based on the setting of the "Build > Oscillator Frequency" project property.

Some older projects may not have "Build > Oscillator Frequency" project property, in which case look for the definition in the "Preprocessor > Preprocessor Definitions" project property on the solution, project or file nodes in the "Common" configuration.