Database location
By default, your database lives safely in your ~/Library/Application Support/OmniFocus/ folder. If for some reason you want to keep it somewhere else, or switch between independent databases, you can do so in the General preferences.
For example, you might like to keep your database on an external drive that you bring with you to the office, or even on a network-accessible drive (though that might be disagreeably slow).
To move your database somewhere else, and make OmniFocus look for it in the new location from now on: Open OmniFocus's General preferences.
In the Database section, click the Move to a New Location button.
Choose a new home for your database.
To choose an existing file to use as your database from now on: Open OmniFocus's General preferences.
In the Database section, click the Select a Different Database button.
Choose another OmniFocus database file to use from now on.