Converting OGG Files
Even though you can now play OGG files in iTunes, you can't play them on your Apple device. This requires an extra step. Apple devices will only accept media files in the default types supported by iTunes.
Select any OGG files in your iTunes library. If you want to select all OGG files at once, right click any of the categories at the top of the iTunes media list, such as Name or Artist, and choose Kind. Click Kind to sort files by file type. This will organize all your OGG files in one group.
Once you've selected the OGG files, select the Advanced menu. Choose Create AAC version. This option may be different depending on your default import settings. For instance, if you import files as MP3, your option will say Create MP3 version. For best results, convert to AAC. You can change your settings at any time by selecting Preferences from the Edit menu. Press Import Settings and select AAC Encoder as your format type.
Though iTunes works great for converting OGG files, so you can use public domain music on your Apple device, you can also use other applications, both free and paid to convert OGG files before you even place them in your iTunes library. If you encounter any problems using OGG files in iTunes, try converting them with another application first.