There is no link between the 40k Universe and Warhammer world. GW said as much a few editions back, and to be honest I hope it stays that way. By thinking that the Warhammer world is just some planet, in some solar system, in some star system, nestled into one of the corners of the vast galaxy somewhat diminishes its importance and uniqueness. However, they both, share certain things, such as elements of one anothers pantheon of gods, High Elves and Dark Elves are very similar to Eldar and Dark Eldar, and they both feature Orc(k)s and Daemons.

Yep, GW did indeed say that there was no link between the two, despite the Old Ones appearing in both.

The Warhammer Fantasy Old Ones came in 'great, interstellar ships' to the Warhammer World, and decided to add it into their plans (whatever those may be). They themselves didn't manipulate the world; they created and gave powers to the Slaan, who manipulated the world's orbit to make it warmer, split continents and created great oceans (known as the 'great pond' to Lizardmen). The North and South Gates were created, and the North gate collapsed either before or after the Old Ones left, resulting in Chaos spilling forth.

In Warhammer 40,000 the Old Ones were a race of great power, but weren't considered god-like as they are in WHFB. They had great technology, but found themselves undone by the Necrontyr and their dark 'gods' (Deciever, Nightbringer etc) and so created psionic races (Eldar) to counter them. But then the Enslaver Plague occured, and bye-bye Old Ones. They died, whereas WHFB Old Ones simply left.

Hope this helps.

As I recall, back in the early '80s the WFB world was located in soem sort of bubble within the Warp (I think it was within the Eye of Terror, but that's kinda hazy). At that time Orks were a symbiote with algae (Read Waagh the Orks, now OOP), not a sentient fungus. Now GW says there is no connection between the two settings. It's their game, I guess they can change their minds.
As to the Old Ones, maybe iy's my perverse idea, but I've always associated them with the Elder Gode of H.P. Lovecraft's Cthulhu mythos. Of cousre I have three Slaan named Choclotl ( the brown one), Wakalotl (the young lieutenent who does most of the dirty work and occasionally gets off his throne) and Itzalotl (the glutton).

Many speculate the Old Ones are the Slann. It makes a lot of sense, but no one is sure. I delve into the Old Ones in my story (See link in sig) but I make no distinction of them as the Slann since nobody is really sure.

The Old Ones created the races of the Eldar, the Krork (Ork), the Humans, and myriad other races, as tools to fight the Necrontyr, but the slaver plague eventually destroyed their civilization and ended the war. They were highly psychic, and masters of warp technology. They were so psychic that much of their technology was warp powered.

i love these guys so much

the current gw statement is that the Old Ones are the same in both games, but the games are both set in different universes both linked (as all universes are) by the warp/realm of chaos

Sigmar was certainly not a Primarch. Firstly because he was born to mortal parents. Secondly because his stature was that of a normal man, he wasn't any bigger or stronger. Thirdly, and probably most importantly is that all 20 Primarchs are said to have fought in the Great Crusade in 40k, that means Sigmar would have ruled an Astartes Legion which he obviously didn't!

If anything I would say the Chaos Gods play a greater role in 40k than Fantasy.

No the Chaos Gods were/are not the Old Ones.

Both media are completely different to appeal to different types of fan.GW are well within their rights to take an excellent piece of fluff material and re-use it if it suits them. After all, that is much easier than creating TWO universe`s worth of fluff. (Not that THAT wouldn`t be fun. Hehehehehehe. *IDEA*)