As it stands, only conjuration and the summoning of daedric princes is the only form of "oblivion magic"(1) has ever extensively made it into the games. However, through lore and quests, other aspects of this branch of magic have been made known to us.

Opening portals to Oblivion:
Three instances of this are known to me, and this process seems to be normally quite complicated.
a) Arena (Using the Staff of Chaos to open a portal to Oblivion to release Uriel Septim, who is being held prisoner)
b ) Redguard (Using N'Gasta's Necromancy Tome to reproduce a ritual to open a portal to Clavicus Vile's realm. You combine Orcs Blood and Amber, Unicorn Horn and Hist Sap, Daedra Heart and Ectoplasm and poor them, in said order, into a chalice.)
c) Oblivion (Where Martin Septim performs a ritual to open a gate to Mankar Camoran's Paradise. In this he requires a Aedric Artifact, a Daedric Artifact, a Great Welkynd Stone and a Great Sigil Stone)

Transporting oneself to Oblivion:
a) Oblivion (Evidently different than opening a portal. This is encountered in Peryite's Quest. It seems to be an attempt to enter an Oblivion realm using astral projection, rather than conventional--though seemingly more difficult--physical means. Perhaps a work-a-round?)
b ) Knights of the Nine (Talo's Blessing)

Daedric Nature:
a) Oblivion (Mankar Camoran and his children have their spirits kept in Oblivion, making the death of their mortal, Mundus bodies inconsequential.)
b ) Knights of the Nine (Umaril the Unfeathered, again, keeping safe his soul in Oblivion)
c) Shivering Isles (Maybe?)

Ingenium:
a) 4th Era lore (What exactly is this? I understand it funnels power from Clavicus' realm in exchange for souls. But what exactly is it?)

Anyway, I could use the help of some lore-ists on this one.

I'm also wondering if there is a standard procedure to opening a gate to the Oblivion realm, as I'm not sure on whether the above examples are considered solid lore or simply lore that has been revised (I'm looking at you Redguard). Does it differ for each prince/realm? Is there a procedure for inter-oblivion travel? Is it different? How do you get back to Tamriel if a gate closes? Is there a different procedure? Lore in the Infernal City seems to suggest its the same or similar...

I'd be happy for some sources (http://www.imperial-library.info/obbooks/liminal_bridges.shtml this one has been mildly helpful)

1) Is there a proper, all encompassing term for this branch of magic? Conjuration, maybe?

EDIT: Stupid sunglasses smiley face. When will you notice the difference between an upper case and a lower case b?

Ooo ouch. 116 views and no reply. I'll bump this once, and then let it die if it comes to it.

Huh, didn't notice this. I would add Sul's travel through Daedric realms to your list, but other than that I have no real comment. Conjuration could fit well enough, if you're conjuring portals. We also don't really know anything about the ingenium :shrug:

http://www.uesp.net/wiki/Oblivion:The_Doors_of_Oblivion can also be useful.

"Liminal Bridges" also describes how to open portals to Oblivion as well. TIL's down for maintenance, so here's the link to the book's page on UESP: http://www.uesp.net/wiki/Lore:Liminal_Bridges

EDIT after reading through the OP again:
Book already mentioned.

conjuration, alteration, Daedric, and speechcraft.

Conjuration is the process of taking something from somewhere and bringing it to you. Not necessarily from oblivion though; The battlemages trapped in Ionith summoned water and Masters of Conjuration can summon eachother's thoughts.

Alteration is used to bend the laws of physics temporarily. this is important because opening a portal to another dimension changes some laws.

Daedric is used because, at least the way the mythic dawn did it, you must have convinced someone in whichever realm of oblivion you are trying to go to to open the door unless you teleport there. (they must write their name on the sigil stone I think) This is also where speechcraft fits in.

by the way, i think Peryite's follwers, in trying to reach his realm, ended up on one of the islands in Mehrune's deadlands. Peryites realm definatley doesn't look like that. And Boethia's shouldn't either but I can't think of an excuse. Perhaps it was one of those realms that Daedra fight for influence over, and Boethia had recently taken it over.

Could have been a plane of least resistance.

Yeah, I understood that Peryite's followers had failed to reach their destination and instead ended up in Dagon's realms. But still, it seems to be a unique way of getting to Oblivion. If the followers would have just opened a portal, they could have just walked to Peryite's realm. But instead they ended up with their souls stuck in Dagon's mini-island realm apart from their bodies. Conversely, in the KotN, the CoC is able to successfully project himself into Oblivion---thanks to Talos--- similarly leaving behind his body to fight Umaril's soul in Oblivion.

I suspect Boethia simply hijacked one of Dagon's island realms for his own gruesome pleasure, which allowed him to keep any pesky heroes from entering his own realm.

Thanks for the replies guys and gals!

And it would seem that Elder Scrolls IV and "Liminal Bridges" does over-write Redguard methods of opening a portal to Oblivion. I guess we could consider Cyrus' method as moot.

Not Oblivion, the Divine Crusader entered the spirit realm. It's different, so I've heard.

You aught to read the book Yar-yulme linked to. There are other methods for reaching Oblivion.

Well naturally what comes to my mind first would be someone skilled at Conjuration. After that Alteration. But youd have to think, any one person, not entirely skilled at any form of magic would be able to somehow creat a portal to/from Oblivion with the right book, i.e: the book you take to Martin for the MQ. But maybe Im also just thinking out loud

I did. I just didn't bother to update my original post. Besides, a lot of what the book talks about are existing holes, nothing you need magic to get through.

What else, besides magic, lets you through? The hole isn't really there, magic requires that one believes there is.

Drop Galerion. He was a fart.

His politics against necromancy was always very short sighted, and his propaganda against him was unjust.

By the way, method from Redguard is mentoned there. As a legend, but it's exist in Oblivion lore.

Not quite. Liminal Bridges talks in a very roundabout way about getting Daedra in.

Getting mortals out is almost trivial, see Doors to Oblivion. You only have to want to be there.

I was under the impression that with Peyrite and Boethia, it was just developer shortcuts. Why develop entire tilesets for stuff you're only going to see once?

Well, this is the real truth. But since what happens in the game is considered lore, then in order for it to fit in properly, new lore-explanations needs to be made to explain developer decisions.

Developer shortcuts make up half the Lore.