Is it OK to substitute one for the other? 
Chives are an herb, green onions are part of...well an onion. They are often used in similar ways and in similar dishes.  You can substitute one for the others in most cases, but I could not say they are interchangeable.  They have distinct flavors and it will change the dish to change the ingredient.  Also do not attempt to replace green onion with dried chives 1-to-1 (or fresh chives for that matter).  Dried chives, not uncommonly found in pre-packaged spice racks, have about as much flavor as straw and will not deliver the desired result (unless you like straw). 