Is there any way I can use TreeSize Free to check how much space my OneDrive files are taking up on the storage?
Assuming that your OneDrive files are synchronized locally, you can use TreeSize Free to both analyze how much space is used up on the storage and locally.
First, select the OneDrive folder under "Home" -> "Select Directory" -> "Documents and Cloud" -> "OneDrive" or by using the directory icon above and navigating to the OneDrive folder that way.
TreeSize Free then scans the OneDrive storage. The "Allocated" column tells you how much space these files take on your local drive while the "Size" column represents the size of these files on the storage.