The Hoops were beaten 5-0 at Loftus Road and Onuoha admits that his performance was below par.
"Collectively we were very poor today," said the 25-year-old via Twitter. "I personally had a really bad second half.
"We appreciate the fans coming out and supporting us. We will do everything we can to make sure days like this don't happen again.
"Well done to Swansea. They deserved their win."
Next up for QPR is a trip to Norwich City, who also lost 5-0 today.