To configure MacPractice to open images in OsiriX, open MacPractice Preferences and select the Digital Radiography panel. Next, select the Image tab and find the Open Images with menu. Select the Other option.
From the resulting menu, select the OsiriX software from the installation location. Generally, this will be the Applications folder.
Select Osirix, and click the Open button. When the window disappears you will see in your Osirix selected in the "Open Images with" menu.
Once this option has been set, double clicking an image in the Digital Radiography ability will result in open the OsiriX software opening to display the image.