MS Word has a native Mail Merge function that works well with MS Excel. Despite the name, Mail Merge works well for longer documents and reports, not just mass mailings.
If you want to use On2Air Docs (an excellent tool!), I believe that the template needs to be a Google Doc, even if the final format is MS Word. So if you need to start with an MS Word template, you might need a different method.
Another consideration is whether your files already exist in the cloud, or if they are only available on your local computer. If they are only on your local computer, that limits your choices.