Does OneSearch include everything the library owns?
No. Not everything that is available from the library can be found in OneSearch. Some databases may have restrictions on them and are not available in OneSearch. In other cases, while a database is indexed in OneSearch, it works better in it's own interface.
OneSearch is a fine place to start a search, particularly if you only need a few resources. However, more in-depth work will require looking in multiple databases or resources.