If you're new to OneDrive sharing is easier than ever. If you're new to Office Online sharing is an easy process too. Learn how here.
Once you've shared a document with others, they'll be able to access it at any time. If you gave them permission, they'll also be able to edit it with Office Online. In fact, multiple people can edit the same shared document at the same time. If someone's editing the document, you'll be able to see that person's name in the top-right corner of the window.
As other people make changes to a document you have open, you'll be able to see the location of their cursors, although you won't be able to see their edits until they save their changes. These changes will be applied to all versions of the document.
This aspect of sharing can be a bit unpredictable. When you're editing at the same time as other people, the changes they make might not appear immediately. You may need to refresh and reopen the document to see their most recent edits.
You can communicate about shared documents by adding document comments. When you comment on a document you've shared with others, they will be able to see and respond to your comment.