Maximara; i also said that dimensional & temporal multiverses are simply different facets of a true multiverse in its entirety. Iow Earth-1 (for example) is both a parallel universe & a divergent timeline. It's a parallel universe in relation to Earths 2, 3, 12, S, X, Prime, etc.. It's a divergent timeline in relation to the earths of; Kamandi, Tommy Tomorrow, the Legion of Super-Heroes, Abra-Kadabra, Reverse-Flash, etc.. As for OA existing only in the Earth-1 universe, i tend to discount/ignore that. The only place that was claimed was in Crisis. According to current theory to travel between divergent timelines you would need to travel through hilbert space. A nigh-infinite dimensional space that contains/separates divergent timelines. Sounds like Hypertime, as described by DC. With some differences, of course. The limbo-like dimension that exists between the universes of the Pre-Crisis DC Multiverse seems more like the higher-dimensional space talked about by m-theory.
The RPG suppliment GUPRS Infinate Worlds is a good source book to help get a handle on all this. It breaks the multiverse down into several distinct catagories:
1) Echo: Effectively your world earlier in time.
2) Close parallels: very similar to your own timeline though there are differences. Earth-B and Earth-E were close parallels to Earth-1.
3) Further parallels: major differences in history. DC's Earth-S and Earth-X were further parallels.
4) High inertia parallel: despite changes in history the world still resembles your own reality. Earth-2 and the unnamed Earth in 'Superman, you're dead, dead, dead!' were High inertia parallels.
5) Myth parallels: a reality where fictional characters or beings realy exist. Earth-2 was a 'myth parallel' to Earth-1.
Some other relevent ideas form GURPS IW:
Reality Shard:Items of great power that can distort the very fabric of reality around them. The Spear of Destiny was a reality shard.
Reality Quake: a mammoth upheaval in the very nature of reality even ltering history itself. One of the earliest modern stories was
Sideways in time written by Murray Leinster in the 1930's.