Omega Dimension

The Omega Dimension is a frozen dimension that serves as a prison.

Overview
The worst criminals from all over Magix are sent to this desolate place, deeply frozen and abandoned, in this supposedly "escape-proof" prison. Tecna was once trapped in the Omega Dimension. Luckily, as Tecna earned her Enchantix and combined it with her technology, making a survival suit, turning ice into food and keeping her warm until Bloom and Sky found her.

Season 3
This place was first introduced at the beginning of Season 3 when the Trix were sent there. Icy was able to free her sisters and herself, thanks to her power over ice. It was there where they first meet Valtor, the main villain in Season 3. To escape, they broke open the portal which was linked to Andros, Aisha's home planet. Rumors spread that if the portal didn't close soon, Andros was going to be destroyed, because the energy of the Omega Dimension and that of Andros are of opposite nature.

The Winx arrived to close it, but it was Tecna, who went inside the vortex generated by the portal, and closed it, earning her Enchantix, but also falling inside in the process. The Winx believed that their fellow friend was dead, but Timmy trusted his instincts, which told him that Tecna was alive.

It was later revealed that she was stuck in the Omega Dimension. When the Winx and the Specialists go there to find Tecna, it's revealed that some prisoners who escaped their ice cells were ruling the planet. Thanks to Bloom, who lead the Ice Serpent, who is the main guardian of the Omega Dimension, towards the prisoners, they were once again frozen. After the Winx leave, no more about the Omega Dimension was heard in that Season. According to Bloom, the frost/guard snakes maintain the magical balance of the Omega Dimension.

Season 4
The Omega Dimension appears again in Season 4. This time, it takes place in the Season 4 finale episode where the Winx and the Wizards have their final battle, and it's also where the Wizards of the Black Circle/Fairy Hunters themselves meet their end. After becoming overpowered, and then struck by the Winx and Nebula's Believix convergence spell and defeated for the final time, Ogron, Anagan, and Gantlos became frozen solid themselves there, then as the ice crumbled all around them, they became never to be seen again after they fall into a ravine, when all the ice around them crumbled because of their actions and the rage of the final battle. Nebula wanted to remain in Omega Dimension as a punishment to herself, because she thought that her wish for revenge against the Wizards had made her evil like the Wizards themselves, but was convinced by Bloom that she was not made evil, and that inside of her, there was still some goodness and light in her heart, then she agrees, and she returned with the Winx out on to the surface to confront Morgana, who then forgave her.

After this, the Winx became reunited with Aisha, then a ray of sunlight appeared there, upon which Brandon noticed that sun rays are rarely seen on the Omega Dimension's surface, and that one appeared as it was a special day. Morgana congratulated the Winx on their courage and defeating the Wizards once and for all.

Trivia

 * Omega is the last letter of the Greek alphabet, denoting the sound ō, and written as "Ω" (capital form), and "ω" (minuscule form). Being the last in the Greek alphabet system, it is usually symbolically used to denote the end of something in contrast to the letter alpha, the first letter of the Greek alphabet, which represents the beginning of something.
 * It is implied by Gantlos that the Omega Dimension is at the end of the Magic Dimension.
 * Obsidian, since evil creatures are imprisoned there and is made of only one material, obsidian stone, is similar to the Omega Dimension where all the worst criminals of the Magic Dimension are sent and which is made entirely of ice.
 * However the Omega Dimension appears to be more mild, as it is frozen one may even survive there and only criminals are sent there while in Obsidian everything that is not evil is destroyed, and even the evil ones suffer, as the three old witches lost their bodies, and not only are the most evil creatures imprisoned there, they are also spawned there, like the Ancestresses were.
 * The guardian of Obsidian, Mandragora, is also a servant of evil, while the guardian of the Omega Dimension, the Ice Serpent, is neutral and is neither good, nor evil.
 * The Omega Dimension is considered a "frozen" planet, but in the last episode of season 4, we can see that there is no ice on the surface of the planet, but instead, there is grass.