Oni (鬼, literally "demon" or "ogre") or Ogres (in FUNimation dub) are a race seems to take care of Afterlife in Dragon Ball Z. King Yemma is the head of all oni, and he also decides which souls go to Hell and which go to Heaven.
Oni do not lie, because King Yemma severely punishes them for it ("will tear my tongue off" in original Japanese, "will be very unhappy" in FUNimation dub).
In Japanese version oni often speak in weird ways, adding unnecessary signature sounds at the end of each sentence.
Oni leading and guiding souls
The first one that is shown in the anime is a blue guy in white shirt and a tie, guiding the line of dead souls with a megaphone. This will be a recurring image of Other World, though the exact appearance of the oni can be different every time. However, most oni are blue, have black hair, one horn and dress neat.
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When King Yemma agrees to send Goku on Snake Way, he provides him with a guide who takes him to the start of the road. The guide seems to be more intelligent and knowledgeable, wears glasses and has two horns. His exact function in Other World is unknown, but later he meets Fortuneteller Baba and gives her Goku's message for Master Roshi (which reaches him via Kami and Yajirobe).
The guide speaks with adding はい (hai, literally "yes" or "indeed") to each sentence.
On his was to King Kai's place, Goku meets a cleaning machine that is driven by another typical oni that gives Goku an unfortunate ride. The driver speaks with adding おに (oni) to each sentence.
Goz and Mez
Goz and Mez (Gozu, Mezu) are two watch-keepers of Hell. Goz is the second strongest oni after King Yemma, and Mez is the second fastest oni after King Yemma. Mez looks just like the guide oni, but his skin is red and he is much more buff (but still wears glasses). They wear t-shirts with "HELL" label on them in matching color. In the English version, HELL is changed to HFIL, which stands for Home for Infinite Losers.
Very much like the driver oni, Goz and Mez speak with おに (oni) in every phrase.
In the movie, Fusion Reborn a teenage Oni named Saike was careless while working the Spirit Laundry Machine (distracted by his death metal music) and forgot to change its tank. The tank overflowed, and Saike was surrounded by thousands of years of spiritual waste. The purplish-gray smoke mutated his body, and turned him into the demon, Janemba. At the end of the film, after Janemba was hit with Gogeta's Soul Punisher, he transformed back into the teenage Saike, who ran away, frightened. Saike has red skin, and two horns.
Another Oni appears in Fusion Reborn, though he is not named. This Oni is the only witness to Saike's transformation into Janemba, and rushes to tell King Yemma, though he is covered in crystal along the way. This Oni has blue skin, and one horn on top of his head. The conversation between them prior to the accident implies that this Oni is Saike's dad.