Oerth

Oerth, the world of the Greyhawk campaign setting, is an alternate Material Plane world parallel to Toril.

Inhabitants
Oerth is home to prominent wizards such as Bigby, Drawmij, Tenser, and Mordenkainen whose spells have spread even to the Forgotten Realms. Mordenkainen is an acquaintance of Elminster Aumar's; the two have met on numerous occasions at Ed Greenwood's house on Earth to exchange spells and lore. Mordenkainen, when he had newly mastered interplanar travel, used to visit Waterdeep with some regularity.

In the book Passage to Dawn by R.A. Salvatore, when Harkle Harpell enters into an unorthodox contest of wizardry with the Sea Sprite's wizard Robillard, Harkle and Robillard cite a number of Oerthly wizards, including Bigby, Melf, Otiluke, and Tenser. "Robillard blanched despite his own impressive display. Of course he knew of Bigby! He was one of the most powerful and impressive wizards of all time, on any world."

The Oerthly bard Gwydiesin of the Cranes has also chatted with Elminster on occasion.

Both Elminster Aumar and Khelben "Blackstaff" Arunsun have spent time on Oerth, and Elminster suspected that Khelben Arunsun the Younger settled there to begin a life away from the elder Khelben.

Relative time
The year 1371 DR appears to be equivalent to 591 in the Common Year timeline used in parts of Oerth, assuming the Forgotten Realms adventure For Duty & Deity and the Planescape adventure Tales From the Infinite Staircase (which were marketed as crossover adventures) both took place in late 570 DR, and assuming that the adventure Die Vecna Die! (which included scenes in the Greyhawk, Ravenloft, and Planescape settings) took place a few months after Tales From the Infinite Staircase and Faction War, which seems evident from the political situation in Sigil.

This is supported by the "Wizards Three" series in Dragon Magazine. In Dragon #185, it's stated that the events of the novel The Parched Sea, which takes place on Toril, have just recently occured. "Novel Ideas" in Dragon #196 places The Parched Sea in 1360 DR. Dragon #185 also places the meeting described in that article prior to the events of the adventure Vecna Lives (which happened in 581 CY according to the Greyhawk supplement The Adventure Begins), which suggests that 1360 DR equals 580 CY.

Spelljammer
In the Spelljammer setting, Realmspace is part of a celestial configuration known as the Triad, which also includes Greyspace, where the world of Oerth is found, and Krynnspace, home of the world of Krynn and the Dragonlance campaign setting. The Phlogiston, a rainbow-colored chaotic liquid that surrounds the spheres, has a strong current that flows from Realmspace directly toward Greyspace, making travel in that direction relatively simple; due to this current, it is impossible to travel directly to Krynnspace from Realmspace. From Greyspace, however, it is a relatively simple matter to travel to either Realmspace or Krynnspace, strong currents leading to both spheres. From Krynnspace one can easily move to Realmspace, but travel directly to Greyspace from Krynnspace is impossible.