Foss, 18, invited the 19-year-old singer and the group on board a bus, which she and over 30 her friends had been travelling on while celebrating the end of their exams, during their stop in Oslo and the pair reportedly hit it off as they travelled through the night partying.
The bus party is a pre-graduation tradition in Norway known as the 'Russ' and is a three-week long event, reported Sun online.
"Harry was overjoyed to get the invite to jump on the bus with screaming fans. There were loads of girls, but he focused on Camilla. It got pretty sweaty in there," a source said.
"Fans are saying they've been seen kissing and she's been spotted at his hotel," the source added.
Styles, who has been pictured with Foss on Twitter, couldn't contain his excitement about the night.
"It was fantastic. I was in that bus for three hours. It was the best time ever. As soon as I got on there were three beers poured over my head and people screaming, 'Welcome to Norway!' I did behave," Styles posted.