What Is The Biggest Difference Between Microsoft BI and Oracle OBIEE?

One of the key differences is cost, Microsoft has both cloud/on prem, which is lesser than OBIEE.. OBIEE Functionalities can be achieved only if you combine SSRS and Power BI. Power BI is still evolving and adding a lot of functionalities. For enterprise, with dashboard, bursting, scheduling, email, export etc.. OBIEE is much better. But for SMB and adhoc and exploratory analysis power BI is much better.

I personally consider Microsoft BI a solution for small to midsize business.
Users with excel experience can relatively easily learn to utilize the BI features.
Unfortunately, it’s not very powerful or user friendly to be considered a self-service tool.

The oracle product line is very powerful and Hyperion is very well known.
Unfortunately, Oracle never integrated all BI tools into a central platform and like Microsoft functionality is spread across multiple tools.
Oracle isn’t very forthcoming with road maps and upgrades / fixes.

If I have to choose between one of them. Oracle wins.

MIcrosoft's Power BI is really vendor agnostic and has a low cost of entry. It can connect to almost any type of data and allows for rich visualizations, reporting and dashboards that are can be fully customized for your needs. When building on Power BI and deploying to PowerBI.com your users can access the information from their phone or tablet via the PowerBI app. Oracle OBIEE is a larger suite of Oracle applications that produces reports, analytics, and integration. I would recommend Power BI for the flexibility, time to implement and overall cost.

Basically, the main difference is that OBIEE is oriented to predefined Analysis versus PowerBI which is related to ad-hoc analysis where you can have self-service BI functionality. If you want to campare an Oracle Tool versus Power BI, you can check Oracle Data Visualizations, which has the same functionality for same goals.