OctoPrint is meant to be accessed through the network primarily. OctoPi does not ship with a setup that will show OctoPrint on an attached display (and considering the amount of various displays out there that would be a nightmare to support too), you'll have to configure that yourself. Keep in mind that OctoPrint's web interface is really not meant to be run on the Pi itself, it is developed to take load off the server to the usually more powerful client and running it on the server could negatively impact your print results.