Issue
Some files or folders are not able to be uploaded to to OneDrive through the web or can't be synced using the OneDrive Sync client. Although there can be issues on both MacOS and Windows, there are some restrictions that may be more prevelant with MacOS.
Solution
A full list of restrictions and limitations can be found in Microsoft's knowledge base.
For MacOS, there are a couple of issues that will be more prevelant: File or folder name and file or folder path length.
File or Folder Name
MacOS has traditionally allowed many characters in their file names that Windows can't use. Since OneDrive is a Microsoft product and base don Microsoft technology, the same rules that limit file names on Windows also effect OneDrive. Any file name that contains any of the following characters will not be synced or can not be uploaded
<, >, :, ", |, ?, *, /, \
File or Folder Path Length
In addition to special characters, Windows has also traditionally had a limit to the length of file path. The path is the location on your hard drive where the file exists. Windows has a limit of 260* characters, OneDrive supports up to 400 characters in most cases and MacOS can support up to 1024 characters or more. Although mostly unlikely, there could be limitations on how long the path is inside of your OneDrive folder that could be causing an issue.
*Technically NTFS itself can support up to 32,000 characters in a full path with 255 characters in each individual component of a path and the file name itself however most applications limit the total path length to 260 characters. This has been a limitation, known as MAX_PATH, in Windows since NTFS was conceived. This will be changed in Windows 10 when the restriction can be lifted in some cases, although most apps won't support longer file paths for a long time, if ever.