The Oppressor One, much like the Skith themselves, was composed of a type of crystal similar to ice. The ship was powered by a fusion core, and had a column of light in its centre that either contained or defined the ship's central computer intelligence, the Mindcore. Areas of the ship contained holding cells, extensive libraries, and laboratories for genetic cataloguing and experimentation. (COMIC: The First)
The Skith used the Oppressor One to visit alien planets. When they found a planet in its original state (undisturbed by other aliens), the Skith would make a detailed catalogue of the planet's species and perform experiments. If the planet was already discovered by aliens prior, the expedition was deemed a failure, and the planet was frozen, with any surviving sentient life converted to Skithself, which would either be collected as samples or be commanded to kill one another.
The Oppressor One landed on Earth in Antarctica in 1915. Its crew conducted genetic experiments on the nearby whales. Shortly after starting, they encountered the Imperial Trans-Antarctic Expedition of Ernest Shackleton, whose ship, the Endurance, was trapped in the nearby ice. They also soon encountered the Tenth Doctor and Martha Jones, who quickly took exception to the work the Skith were doing. The Skith Leader's interrogation of the Doctor led him to the conclusion that they were not the first visitors to Earth, making their trip there a failure. The Leader started the planetary conversion, but the Doctor, with Shackleton's help, sent the Oppressor One into the sun. (COMIC: The First)