Operating Expense Ratio is an indicator of how well a company is managing the ongoing costs of operating the business. It takes the operating expenditure (OPEX) and divides them by sales in a given period.
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.