Yes, Hopper contains one-way flight options!
On iOS: Tap on the Search tab, then tap either the To or From search fields. The option to toggle between One Way and Round Trip options will appear above. Simply select the option you would prefer and search away.
On Android: Tap on the Search tab and enter the To and From fields, then tap on the One Way option on the bottom of the screen.
Please note that multi-city search functionality is not available in the Hopper app at this time. You can, however, search a series of one-way segments to make up a multi-city itinerary.