Former employers usually refer to former employers. For example, if you are looking for a new job after leaving your job, the new company will ask you why you left your previous company. At this time, you can use "former employer" to refer to your former employer.
An old boss refers to a long-term cooperative employer, and can also be used to refer to an enterprise or organization in his hometown.
Generally speaking, the former owner and the old owner can be common in some cases, but they are not exactly the same.