File associations in Mac OS X are essentially tags that the operating system applies to each of the different file types on your Mac’s hard drive. When you double-click to open a file, these tags inform OS X which application to launch.

When you first start using Mac OS X, many of these associations are for the software that comes built-in or pre-installed on your hard drive, but you may have other programs installed that you would prefer to automatically open for certain files. While you could Ctrl-Click each file and select Open With, in the long run it is easier to change the default file associations.

1.) Open a Finder window containing file types whose default association you wish to change. In this example, we have a selection of document files:

Documents in a Finder Window on OS X.png

2.) Select the file you wish to change, and Ctrl-Click (or Right Click) on it, and then select Get Info:

The Finder Contextual Menu in OS X.png

3.) In this example, we can see that the default application for opening a .doc file on this Mac is Apple’s Pages:

Get Info Window for a Document File in OS X.png

4.) Go to the Open With section and choose a different application from the drop-down menu:

Changing the File Association in OS X.png

You can either select an application from the list, or choose Other… to navigate to an application on your hard drive.

5.) We’ve selected Open Office to open this .doc in future:

Document File Association Changed in OS X.png

6.) To alter the file association for all files of the same type on your Mac to the new program, press the Change All… button. A dialog box will appear asking you to confirm that you wish to change all similar files to open with that application. Click on Continue:

Changing Associations for all Files of Similar Type.png

OS X will now alter the file association for all other files of the same type on your Mac to the new default application, so that whenever you double-click a similar file in future it will open within that program.