Recently I was asked the question “How do you use Power Pivot with SharePoint Online" and the answer is to use Excel 2013 or later, it will get you a basic level of use with Power Pivot and SPO (E1 – E5 License) as suggested by this article (See all Stages but the last that discusses Power BI) https://support.office.com/en-us/article/BI-capabilities-in-Excel-and-Office-365-26c0548e-124c-4fd3-aab3-5f64568cb743. You will need to install the add-in which you can do by following the steps in this article https://support.office.com/en-us/article/Start-the-Power-Pivot-in-Microsoft-Excel-add-in-a891a66d-36e3-43fc-81e8-fc4798f39ea8.
Here are two articles that can give you a good idea of where Power Pivot started and where it is going http://www.microsofttrends.com/2014/02/15/powerview-vs-powerpivot-vs-power-bi/ and https://blogs.office.com/2016/03/14/new-feature-updates-for-power-pivot-in-excel-2016/
Another great tool in the BI stack is Power View, check out this article and see the awesome visualizations that you can create and Share using it with SPO https://support.office.com/en-us/article/Power-View-in-Excel-on-Office-365-or-in-SharePoint-Server-99127D68-D7B7-42D0-A4EE-20E2D9897472
Now depending on you license type there might be more goodness that you can enjoy, if you have the E5 license you get Power BI which is the next level of Business Intelligence tools beyond Power Pivot and Power View, here are some resources to help you learn more. If you have any questions please let me know.
Power BI guided learning
Power BI Documentation
Power BI Blog
Power BI tutorials
Have fun out there