The book I'm using as a resource to add jQuery to our SharePoint Online site discusses editing the Seattle.html and Oslo.html files in SharePoint designer. However, those files are not appearing in designer. How can I view these files? If I cannot view them, should I be modifying my html/css/jQuery in another way?
Typically, i find the oslo.master or seattle.master and edit the master page, you can find in your SharePoint Design Master Page on navigation menu, this option show all masterpages in your site, and click on seattle or oslo and edit file in advanced mode. I strongly advise you to make a backup of one of the two and modify your backup.