A COO is great in a large company, where the CEO can be more hands-off and have a more representative role. In startups or small companies it's frankly a waste and just leads to very conflicting sets of responsibilities and infighting. Been experiencing this myself.
As a COO or as a CEO?
Tim Cook was COO of Apple before taking over as CEO. Steve Jobs may have help bring the iPod, iPhone and iPad to the world. But someone else had to figure out how to get those millions of devices into hands of customers.
A COO is in charge of daily operations of the company and reports to the CEO. A famous COO would be Sheryl Sandberg at Facebook.
Daily operations as in "make sure the site is not down and make sure everybody in the office is doing something productive" ?
the COO varies based of the need of the CEO and their working relationship