Fans trying to access the three websites selling the tickets — Ticketmaster, See Tickets and Gigsandtours — reported issues including error messages and being kicked off before they could purchase tickets.
It was expected that more than a million tickets for the band's gigs would sell out within minutes. Instead, the band announced all the tickets in Britain had been sold 10 hours later after many fans had spent the day in online queues.
"There has got to be a fairer, simpler, more efficient way of selling tickets," said Dan Walker, the British TV presenter, on X.
"In the queue, out of the queue, refresh / don't refresh, wait in line, back of the line."
Ticketmaster earlier said its website has not crashed and the queue was moving along as fans bought tickets.
Gigsandtours thanked people for their patience and said there had been "extremely high demand".
At the same time, some tickets were relisted on resale sites such as Viagogo for as much as 8000 pounds ($A15,529).
Oasis announced 17 shows in the United Kingdom and Ireland, with the first due to take place in Cardiff in July 2025, followed by nights in Manchester – where the band was formed in 1991 – London, Edinburgh and Dublin.
The group, whose debut album Definitely Maybe was released 30 years ago, split in 2009 when lead guitarist and main songwriter Noel Gallagher said he could no longer work with his younger brother, Liam, the band's main singer.
At its peak in the 1990s, Oasis exemplified the soaring appeal of Britpop, with hits like Wonderwall, Live Forever and Champagne Supernova.
But Noel and Liam were often in conflict and their strained relationship finally snapped in 2009 as they prepared to play a Paris gig.
Since their split, both brothers have continued their musical careers, but always against the backdrop of calls from fans for the band to reunite.