The office said the figure, estimated for the year ending March 2015, represents a "statistically significant increase" from the year ending March 2014 net migration estimate of 236,000.
"This continues the generally increasing trend in net migration since the recent low of 154,000 in [year ending] September 2012," the ONS said.
In May, the ONS said net migration in 2014 had reached a 10-year 318,000 high.
The BBC cited Immigration Minister James Brokenshire as saying the latest ONS data was "deeply disappointing."
British Prime Minister David Cameron and his Conservative Party won this year’s general elections on a campaign pledge to reduce net migration to the UK to below 100,000.
Net migration is calculated by subtracting the number of those leaving the country from the number of those arriving.
The ONS estimated that immigration to the UK rose to 636,000 this year from 552,000 in 2014. Immigration to the United Kingdom from within the European Union rose to 269,000 and to 284,000 from outside the EU, it added.
