Which phrase is correct to mean"the old company where one used to work"?
previous company
or
former company

Question

Which phrase is correct to mean"the old company where one used to work"?
previous company  
or 
former company

Answer

Previous and former are both correct; however, former has a negative connotation, so you'd want to use previous.

Answer

Both are correct~although if you say, "the company for which I used to work..." people also understand (because of "used to")

Answer

The old company I used to work at = my previous/former employer

In your example, only "previous" is correct.

Answer

I guess probably you can use "previous employer". Also looking forward to answers from native speakers.