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.
If you want to stick with classic pages permanently (disable modern site pages) - it's done via
"Site settings" -> "Manage site Features" -> "Site Pages" Deactivate.
then from site pages library settings click "Change new button order and default content type"
and deselect "Visible" at "Site pages".