Oberon's character designer and artist is mangaka Umino Chica, whom you may recognize from her manga Honey and Clover and Sangatsu no Lion.

Nasu, the original creator of the Fate series who also wrote Avalon Le Fae, met Umino through a mutual acquiantance. It was during this meeting that he proposed the idea of working together on a project, which she agreed to. This was what led to him assigning her Oberon to design, specifically requesting he be drawn in her signature style, rather than a style more typical of FGO.
Oberon's in-game art, as well, was Umino Chica's first work in digital art. He was drawn in Procreate with about 10 layers.

(Click on an image to increase its size!)

In-game Ascension Stages

Stage 1

Stage 2

Stage 3

Stage 4

His miniature "Robin Goodfellow" form.

CGs from the Avalon Le Fae main story

The Abyssal Worm

Art by other artists, used in official sources

Saint Graph from the 2022 April Fools event, Fate/Pixel Wars
(Artist: Riyo)

Chibi art from the 2023 April Fools event, Fate/Grail League
(Artist: TBA)