Oangband, short for Opinion angband, was one of the major variants during the prolific era of variants contemporary to the late 2.x versions of angband.
It is considered one of the more difficult variants. Loot is rather stingy even by the standards of the time and monster AI an improvement over its contemporaries. One of the best known features is the so-called 'O-combat' system, which has been incorporated into or inherited by other variants, notably FAangband. There are many new monsters that pose significantly more risk than the contemporary Vanilla roster, including the notorious Crows of Durthang, added to ComPosband as homage, as well as rabid weasels and many other worse things.
Classes were redesigned for Oangband, providing some inspiration for the class list in Angband 4.2.0 - there are four realms of magic, life, death, nature and arcane, each represented by a pure caster and hybrid class. Each character can select special abilities which provide advantages in specific areas, with availability depending on class and race, which adds an element of character customisation missing from Angband itself. New Races include Maia and Shadow Fairy. Expanded dungeon terrain including water tiles and trees make an appearance here, absent from Vanilla Angband until their introduction in 4.2.0 inspired partly by Oangband.
There is an interactive quest which players can spontaneously stumble upon during their delvings, which is unique to this variant and lends a touch of life to the dungeon.