"Omnioculars. You can replay action … slow everything down … and they flash up a play-by-play breakdown if you need it."
-- an eager souvenir seller at the Quidditch World Cup (GF7)
Omnioculars look like brass binoculars, except that they are covered in all sorts of weird knobs and dials.
They can be set to replay action, zoom, slow down play, and overlay magical words that provide commentary in purple letters (GF7).
Harry Potter buys a pair for himself and his friends Ron Weasley and Hermione Granger from the souvenir sellers at the 1994 Quidditch World Cup, promising no Christmas or birthday presents “for about ten years” as they are so expensive (GF7). Later, during the team mascot display, Ron thinks that he has repaid Harry for them (GF8), but is disappointed on learning that what he used to pay was leprechaun gold, which eventually disappears (GF28).
Because the Omnioculars can be set to watch on slow-motion or rolled backward to replay moves, Harry discovers that it is possible to miss what is going on in “real time” during the match. Ron initially uses his pair Omnioculars to watch and re-watch a wizard in the crowd picking his nose…. (GF8).
Omniocular footage was checked after the USA vs Jamaica Quidditch match during the Quidditch World Cup held in 2014, in order to determine whether Jamaica’s Keeper had been hexed by a member of the crowd (he wasn’t) (QWC).
The referee of the final match between Brazil and Bulgaria in the 2014 Quidditch World Cup also checked Omniocular footage to see whether a foul had been committed by Brazilian Beater Santos on Bulgaria’s Viktor Krum (it hadn’t) (QWC).
A combination of omniscient and binoculars: "omni" = "all"; "ocul" = "eye" (Latin)
Compared to the cost of Harry's wand three years before, which only cost 7 Galleons (PS5), they seem quite expensive.
There is no information on how long the magic in the device continues to function. However, in spite of the cost of 10 Galleons in 1994, and a probable price rise, they are still popular with fans by the time of the 2014 Quidditch World Cup:
"Mere minutes after play resumes, Krum and Silva are rocketing suddenly upwards - five thousand Omnioculars follow the pair into the dazzling Argentinian sun...." (QWC).
From the Web
Writing by J K Rowling on Imgur (Daily Prophet coverage of the 2014 Quidditch World Cup, originally from Pottermore): Match reports and articles from the 2014 Quidditch World Cup
AnyOneThing blog: Omnioculars are Real?
Harry Potter Wiki: