The official campaigns are the three campaigns created by BioWare that were included in Neverwinter Nights and the two expansion packs. When used in the singular, the term typically refers to just the original campaign, instead of all three campaigns collectively. This ambiguity is further enhanced by the acronym "OC", which is used for both "official campaign(s)" and "original campaign".
Although the official campaigns have most often been played with only one player (and are sometimes even referred to as "the single player campaigns"), two of them have been designed with full multiplayer support. In contrast, multiplayer support within the Hordes of the Underdark campaign was dropped midway through its development. However, those willing to put up with a few anomalies and workarounds still can (and have) play through this campaign with multiple players.
The intended progression of the official campaigns is that a player completes the original campaign with one player character, then completes the Shadows of Undrentide (SoU) then Hordes of the Underdark (HotU) campaigns with a second character (starting at level 1 in the SoU campaign). The HotU campaign, in particular, features dialog that assumes the player's character did not participate in the original campaign, yet assumes that the player does not object to reading some details about the ending of that campaign.
Once an official campaign has been played through, it becomes "unlocked", allowing new games to be started at the beginning of any chapter of the campaign and allowing the modules of the campaign to be viewed within the Toolset.
- HotU campaign multiplayer workaround thread, BioWare forums (may contain spoilers)