One of the New Series' most popular monsters, the Ood, are scheduled to reappear in the Fourth Series, according to the Times Online and confirmed on the Official Site.
The Tenth Doctor (David Tennant) and Donna (Catherine Tate) will encounter the fearsome aliens in an episode called "Planet of the Ood".
Show runner Russell T. Davies commented: "We're delighted the Ood are making a welcome return to Doctor Who. They were last seen falling into a black hole back in series two and we think it's only fair for the viewers to find out what the Ood have to say for themselves. Donna is certainly in for a shock."
According to the article, the episode will delve into the Oods' origins. No word yet on when in the series this episode occurs.