Where are the real aliens in Oblivion?

There never were any (bilogical) aliens at all, only the TET. The message from Titan was from the TET alone, a completely artificial entity. The TET then attacked and devastated earth in order to drain its resources. The whole story about an alien attack that is told to Jack and Victoria (and all the others) was just a staging to keep him maintaining the drones and stay away from the remaining human resistance, which he thought to be the alien scavengers. The TET is the "real alien" for that matter.

As to the question who in turn created the TET (and maybe this is what you actually meant with the question). This is never adressed in the movie at all and neither are we supposed to know anything about it. All we are supposed to know is the TET alone and the fact that we don't have any background information about it adds to a large degree to its fascination. It is a completely autonomous entity appearing out of nowhere, a living machine and thus not any less "real" than you and me.

The director Joseph Korsinski answers the question in the link below. Basically he explains that the distance from the Tet's unknown solar system/star to Earth is so vast its unlikely that a living being could make the journey. Therefor an unmanned vehicle was more likely. It took the Odyssey at least 39 days to reach the Tet (in our own solar system) and the crew needed to be sleeping to complete that journey. Logically, I was also thinking whatever intelligence created the Tet and sent it unmanned to take over a whole planet, clone humans and design and build outpost on Earth, must have the capability to send backup. The Tet by any planetary standards appears to be very important and very expensive. It appeared to be on a mining mission. However based on the directors answer and Morgan Freeman's monologue around 1hr and 32 mins into the movie the Tet may be just be a digital community or single entity. Think the Matrix learns space travel. Which might make a nice prequel. I wonder if the Odyssey was on a mission to learn how to exploit Saturn's resources?

Check out http://www.cinemablend.com/new/Oblivion-Spoilers-Director-Joseph-Kosinski-Answers-Your-Burning-Questions-37271.html

Simply put, Tet made an environmentally compliant invasion force of simple-minded (and presumably cheap) "Army" Jacks and unleashed them after the moon was destroyed.

Phase II involved hunting dedicated survivors and protecting the hydrogen convertrers, which required more advanced and creative Jack Clones to keep the more deadly and resource-intensive drones online.

Tet probably had a learning curve to deal with the more advanced Jacks and used companionship, sex, and the "Scav" deception to keep these newer Jacks compliant.

Whatever created Tet never invaded earth. Tet is a giant military drone that used human clones as bullets.

From my post below: The thought was that the AI (Artificial Intelligence) theory was from another world of humans where the TET evolved from, humans ahead that of earth time. Earth obviously would be decades away from AI in 2017, a plausible prequel/sequel. These aliens were not of organic matter, they're obviously machines. Therefore, they know humans and know how to clone them. Then use the clones to do their cheap labor. Apparently, it's cheaper to make humans than create drone repairman. It seemed that the TET was limited in making parts to repair their broken down drones as Vika was told by the TET rep.  The TET has diminishing resources and therefore all the Jacks and Vikas were created from 'local resources' to continue the ops. Just my opinion.