With OneDrive now being able to store your music collection in the cloud the Xbox Music app has been updated to improve the link between the services. When you select a playlist to keep offline it will automatically download all streaming tracks to your phone whether they be from Music Pass or OneDrive so it doesn’t matter which way you store your music the playlist will still work.
Also the app has a better explanation of how to get started with music stored on OneDrive.
Via Xbox User Voice:
A new release is available today for the Xbox Music app on Windows Phone 8.1. This release (version 2.6.674.0) addresses a number of customer issues including:
Better explanation of how to get started with OneDrive in the Music App on Windows Phone 8.1
When you set a playlist to ‘Keep playlist offline’ it will automatically download all streaming tracks, whether from Music Pass or OneDrive