In a statement made at 3.30am, O2 said its 4G network was back up and running.
It follows a 24 hour service outage that left millions of O2 customers across the UK fuming.
"We can now report that our 4G network has been restored," a spokesperson said.
"Our technical teams will continue to monitor service performance closely over the next few days to ensure we remain stable.
"A review will be carried out with Ericsson to understand fully what happened.
"We'd like to thank our customers for their patience during the loss of service on Thursday December 6 and we're sorry for any impact the issue may have caused."
But the provider has admitted that a "small proportion" of customers are reporting problems sending text messages.
An O2 spokesperson said: "A small proportion of customers have reported issues sending text messages.
"We can see that the vast majority are being sent and received successfully and our technical teams are investigating. Our 3G and 4G data services are performing as normal."
One O2 customer wrote on Twitter: "Still getting a failed message every time I try and send a text."
Another Twitter user called Mark, a customer of GiffGiff which piggyback's on O2's network, said: "Still having issues with the network, I'm on GiffGaff network which uses O2 and I'm having probs sending text messages.
"My phone just keeps failed to send every time I send an SMS. Calls have interference, data is ok."
Another GiffGaff user called Anne wrote: "After the situation yesterday when the data went down, I cannot send texts normally.
"It says 'not delivered' on my phone – but to the recipient, it spams them with the same text I sent. I have an iPhone 6s and I have the £20 plan. Please help – thank you!"
While Tesco Mobile customers have reported similar problems. One user wrote: "Still unable to send text or make phone calls. When will that be resolved???"
Another said: "My texts are being sent multiple times and are coming out of my allowance. When will text messages be sorted? Will Tesco customers get the same compensation as O2 customers?"
Someonelse wrote: "So apparently @o2 services have been restored. U ok hun?
"Because I don’t have mine back I’ve reset my phone so many times since the news came out that it’s been restored and still no 4g? #IWantMyData"
Meanwhile, another user said: "@O2 signal and data back but still no 4G in Antrim?
"Is this to be expected? Status checker says I should be 100%"
The Sun has contacted O2 which says the network is back up and running, but advises customers who are still experiencing issues to restart their phones.