What are the differences between seattle and oslo master pages in SharePoint?

 Ans:- 

Seattle:

1.  Seattle is designed for intranet team collaboration portal with more features such as  social, content\site navigation and management,etc.
2. There is a Left navigation is exist and we can set Top navigation for top level sites and sub sites

Oslo:

1.  Oslo master page is designed for publishing site which focus on page layout and content rendering.
2. moving the left navigation to the top, and removing the top navigation all together. Oslo has a wider layout for your content.