If you are adding a page to your Office 365 site via gerbox->"Add a page"
and a "modern-style" SharePoint page is created,
you still can have "classic" experience, i.e. Wiki pages or WebPart pages.
Just go to your pages library via "Site Content" -> "Site Pages",
then click "Files" tab and select "New Document" with dropdown option,
and this is where you can choose what kind of page to create.