The Office 365 files and folders you store in OneDrive for Business are private until you decide to share them. You can share a file or folder by emailing a link to a recipient. You can also share a file or folder by copying a link and sending it to a recipient. When you share files and folders, you can decide whether to let people edit or just view them.
Microsoft has extensive documentation on how to share files and folders from OneDrive for Business located on their support site.