OceanWP’s page layout section is located here in the Customizer.
WP admin dashboard > Appearance > Customize > General Options > General Styles > Pages
Full width and boxed layout is pretty much self explanatory but OceanWP has a layout option called Separate that isn’t described in the documention.
In short the Separate layout makes your content boxed but your header/footer are full width. Hope that saves you minutes of Googling.