The pop star said it was a "spur of the moment" decision which was an “inexcusable error of judgment”.
Scotland Yard confirmed that officers had been called to an address in Notting Hill, west London, on Saturday night amid a breach of Covid regulations.
It follows reports of other gatherings organised by celebrities, prompting a reminder from Downing Street that coronavirus rules apply to everyone.
In a post on her Instagram story, Ora said: “Hello all, I attended a small gathering with some friends to celebrate my 30th birthday.
"It was a spur of the moment decision made with the misguided view that we were coming out of lockdown and this would be OK…
“I’m deeply sorry for breaking the rules and in turn understand that this puts people at risk.
"This was a serious and inexcusable error of judgment. Given the restrictions, I realise how irresponsible these actions were and I take full responsibility.
“I feel particularly embarrassed knowing first-hand how hard people have worked to combat this terrible illness and being fully aware of the sacrifices that people and businesses have made to help keep us all safe.
"Even though this won’t make it right, I want to sincerely apologise."
Downing Street stressed that coronavirus rules apply to celebrities along with everyone else and that enforcement is a matter for the police.
The prime minister’s official spokesman said: “It’s important that everybody in society sets an example by following the rules – that is for every member of the public, including celebrities.”
“Throughout the pandemic we’ve been clear that it’s vital for everyone to abide by the rules in order to suppress the virus and reduce the transmission, but enforcement matter is for the police.
“The Prime Minister has been clear of the need for everybody across the country to continue to abide by the rules in order to reduce the transmission of the virus, but it’s up to police to decide what action to take.”