Shortly thereafter the bubbly network eased the panic by assuring customers the new iPhones would work on its newly-launched next-gen network, with more details forthcoming.
Some of those details have emerged on Wednesday and, unfortunately for the network and its customers, the new iPhones won't work on O2's new 4G network for 'weeks' after the September 20 release date.
'Within the coming weeks'
In its 'The Blue' blog, O2 admitted that it wouldn't be able to get 4G up and running on phones straight away, due to a settings issue.
We got hold of an O2 spokesperson, who gave TechRadar the following statement: "The iPhone 5C and 5S will ultimately work on O2's 4G network, but we are currently waiting for Apple to enable the carrier bundle for our network.
"We've been advised this will be in the coming weeks.
"We want to be transparent about this with customers, so we are giving £5 off, and clearly they'll still have 3G connectivity until the carrier bundle is enabled to allow 4G [for the new iPhones] on our network."
The mention of 'weeks' will surely have some users worried that this 4G shortage on their new device will last longer than a month, meaning the £5 reduction will rankle.
Other network sources have claimed the delay could be longer to TechRadar, but O2 apparently isn't worried, telling us 4G will certainly be enabled on its 5C and 5S models within a month.
In slightly better news, the network has confirmed that the iPhone 5S will be available on its O2 Refresh tariffs, which enable users to pay slightly more upfront to get a new phone mid-contract. The Refresh deals will start at £119 for the 16GB handset.