What is a legal body called that own stock in several companies and so controls those companies?

Holding Company

Explanation:

A Holding Company is a company that enters into a limited partnership by owing a common stock (voting share) in a different company (subsidiary company). This is a type of parent company that indirectly exercises control over another company's operations. This situation is what you call an "oversight capacity."

Although the holding company is not that active in the operations of the other company, it has the capacity to control those who are managing the company it owns. This means it can hire and fire people.

Thus, the answer is "holding company.