OMG Yes, it all makes sense now.
Alright, here we go:
Two ancient races, Oocca and Shiekah. (Given)
Oocca, or Oocca'ish people from the sky, precursors of the Hylians. (Twilight Princess - Shad says it.)
Sheikah, dark ancient tribe. After a betrayal of the Hylian royal family, the tear was added to the Sheikah eye symbol to symbolize their sorrow. (Though that's sourced from a non-canonical OoT manga.)
Messenger to the heavens (SS Link), partakes in some sort of event during Skyward Sword where the fate of the Sheikah, Oocca and Hylians is decided. (My own speculation)
The Oocca also created the Temple of Time, and had some involvement with the creation of the Master Sword (Twilight Princess - Shad/Ooccoo say so, and it's the premise of Skyward Sword.
Master Sword/Skyward Sword aids Link in his adventure during the game (duh) which, once again will probably decide the fate of the 3 races. (Speculation)
The Sheikah probably come off the worse from this and slowly die out, leaving only a few survivors, and probably turn to protecting the Hylian Royal Family. (Ocarina Of Time - Impa tells Link).
Ok, this is where it gets very speculative:
Maybe there were two different factions to the Sheikah, one evil (Interlopers) and maybe one that is noble (Impa's Sheikahs).
Or maybe they're mutually exclusive, Interlopers and Sheikah.
Maybe the Oocca are god-like.
Maybe the Gorons, Zoras, Gerudo, Kokiri have something to do with this.
I dunno. But what I first stated is established fact. bar the Sheikah Eye Symbol stuff. And that last bit can probably be taken as very lazy fanfic. Speculate away!