We have a new feature that allows you to search for oneshots. Go to the "Series Info" page and scroll all the way down to sort by oneshots. You can also search for genres at the same time. The same is for completed series (in Japan, not scanlations).
With the way the site works, this status is determined under the "Status" heading on any series page. If the key word "oneshot" or "complete" is within that section, then it is sorted that way. (Also lists the number of chapters/volumes of that series.) Because series information is incomplete sometimes, if you know of a oneshot or completed series that is not listed as such, please change the information by registering and changing the information.