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Oblivion Gates are the gateways created between Oblivion and Tamriel. They are magical portals that facilitate travel between the two planes. They are made by Daedric worshippers in an effort to essentially merge the worlds, allowing the Daedra to roam Tamriel and rule it.

Background

Closing Oblivion Gates is one of the main tasks in The Elder Scrolls IV: Oblivion. They are found throughout Tamriel and normally have several leveled Daedra guarding them. Once you enter the gate you'll find yourself in a place where you have no friends at all, so be sure to carry enough potions and Repair Hammers with you. Don't forget to rest regularly, especially when your health is low. To close the Gate, you must remove the Sigil Stone that keeps the Gates functioning; these are found in the largest accessible tower in the plane of Oblivion that the Gate led to. Accessing some of these towers may be very straightforward, others may require opening gates (switches can be found at the top of other, smaller towers), using a bridge from one of the smaller towers, or navigating through mazes of paths and caves. Do not be afraid to jump or walk over narrow streams of lava, as it takes a little time to kill you, and it may be necessary for some maps. Despite the relatively small size of the zones, it may take hours to finish the more complicated ones.

Oblivion Crisis

During the Oblivion Crisis all of Tamriel was attacked by Mehrunes Dagons Daedric army. Oblivion gates opened throughout all of Tamriel, many citys were destroyed untill the Martin Septim summoned the Avatar of Akatosh and defeated Mehrunes Dagon during the Battle of the Imperial City.

When getting close to an Oblivion Gate, the sky will turn red and you will hear thunder.

Gate Locations

Oblivion Gates appear pseudo-randomly throughout Cyrodiil, based on how far you have progressed along the Main Quest line. There are 100 distinct locations where a gate can appear, and as each gate is closed that particular location will never spawn a new gate. (Gate re-opening can be turned on via console commands, but is off by default.) However, no more than 60 separate gates will appear during a single game, including 10 gates which always open in designated spots.

There are two gates that open on top of wayshrines. The first opens and completeley buries the Wayshrine of Mara NW of Anvil. The second one opens on top of the Wayshrine of Kynareth, breaking it in half, east of Skingrad at the top of a steep hill down the gold road, however this wayshrine can still be used by activating one of the two halves.

When entering an Oblivion Gate, there are several exterior map templates randomly chosen from, as well as several interior maps for the towers and caves, which allows for different map configurations for each gate. The ten guaranteed gate locations lead to a specifically chosen Oblivion map; five gates lead to a specific one of the random worlds, and the other five have a custom Oblivion map with random towers.

Static Gate Locations

These gates always spawn in the same location, and are triggered as soon as you complete a part of the main quest line. Included in this list are the only three gates that are required to complete the main quest. These gates are:

Kvatch Gate: Opens outside Kvatch as soon as you leave the sewers. Leads to custom Kvatch Gate world.
Bruma Gate: Opens outside Bruma once you return a Daedric artifcact to Martin. Jauffre sends you to close this gate. Leads to custom Bruma Gate world.
Bruma Great Gate: Opens in a field southeast of Bruma, once you arrive there with Martin. (Note that other Oblivion Gates open during this quest, but those cannot be entered). Leads to custom Great Gate world.

Six of the remaining gates are part of the Allies for Bruma quest, one per city that may send allies. These open as soon as you steal the Mysterium Xarxes from the Dagon Shrine. For each city, you need to close the nearby gate before the Count or Countess will risk sending his/her soldiers away to Bruma:

Cheydinhal Gate: Leads to custom Cheydinal Gate world (and The Wayward Knight quest).
Anvil Gate: Leads to map with the Sigil Tower atop an unclimbable mountain, which requires traversing two sets of tunnels to reach.
Bravil Gate: Leads to world with single war gate between the Oblivion Gate and Sigil tower, which is opened from one of two peripheral towers.
Chorrol Gate: Leads to world with six "natural disaster" towers and four war gates.
Leyawiin Gate: Leads to world with dead horse and ruined bridge just inside the gate.
Skingrad Gate: Leads to world with two side towers and retractable bridges.

Finally, there is always an Oblivion Gate near Fort Sutch, which opens at the same time as the city gates; this gate randomly leads to one of four Oblivion worlds (Anvil, Chorrol, Leyawiin, or Skingrad gates).

Random Gates

In addition to these ten gates, up to 50 out of the 90 remaining gate locations will spawn a gate as you progress through the main quest. These gates do not spawn all at once, but appear to spawn as you near their location. For example, you could pass an Oblivion Gate at one location, reload an earlier saved game, and the gate may no longer be at that location. Once a gate spawns and is closed, the dead gate remains present for the rest of the game, and no other gate will spawn there. (Until the end of the main quest, oblivion gates can reopen).

The number of gates, and the odds of a gate appearing, depend on how far along the main quest line you are. As soon as you rescue Martin from the Kvatch Gate, up to 25 random gates will open. As you near each possible location, there is a 25% chance that a new one will spawn. At the same time that the fixed city gates open, the number of possible random gates increases to 50, its highest point. In addition, there is now a 50% chance of any given gate location spawning a gate. Once you have retrieved the amulet of kings from Paradise, the number of gates reduces to 20, with a 20% chance of a new one spawning. Note that the maximum number of open gates also includes dead gates you have already closed; that is, if you have closed more than 20 gates by the time you return the amulet, no more will spawn.

Once you finish the main quest, the maximum number of open gates, and the odds of a new gate opening, both drop to zero. From this point, you would need to use the console to force any more gates to open.

Below is an incomplete list of locations where a gate can open, along with a partial map to the right. (Please feel free to add locations to this list as they are discovered). The list is organized based on the nearest city to the gate location.

Anvil

Far NNW of Anvil, near a Daedric Shrine
Southeast of Anvil, at the top of a hill, nearby the Gweden Farm
West of Anvil
Just NW of Anvil, smashes into the Wayshrine of Mara that's south of Crowhaven
Far Southeast of Anvil, along the Strid River, close to the little island
Fort Sutch (talk to the Imperial Guard Captain optional)

Bruma

from Bruma & from Gnoll mountain
East, about half-way to the Temple of the Ancestor Moths, near the "n" in " The Joroll Mountains" on your map.
West from Bruma, just south to Applewatch