The winner of the bout will earn a ranking among the top 200 middleweights in the world.
O'Brien has had five wins, a draw and a loss as a professional while Nawai has had 11 wins, eight of them by knockout.
A former light-middleweight, O'Brien, is relishing the move up to middleweight.
"I'll get to eat breakfast before I weigh in from now on and I'll be happy about that. During my light middleweight days I often had to survive on cashew nuts, chewing gum, veges and fruit," O'Brien recalled.
The former holder of the New Zealand Professional Boxing Association's (NZPBA) light-middleweight title, O'Brien, is also enjoying life as a fulltime professional, a lifestyle change he made in December last year after ending a four-year retirement.
"I'm the fittest I've ever been because I'm training every day, often two or three times each day and pushing the body extra hard. It's unreal."
As part of his preparation for his Fiji assignment O'Brien has done more than 100 rounds of sparring with the likes of fellow Bay professional Tipene Maniapoto and New Zealand super middleweight champion Jonathan Taylor of Tauranga.
"A lot of fighters take on bunnies and journeymen. I want to take on hard fighters like Silevini, who has heavy hands," O'Brien said.
The Ocean Spa, Modern Yoga and Najala Massage-sponsored O'Brien last fought in March when he scored a split decision victory against Wellingtonian Shiva Mishra. He was hoping to take on Tauranga-based professional Gunnar Jackson last month but the fight didn't eventuate.
Another of Langdon's professionals Toa Leutele, 24, will head to Australia for an eight-man heavyweight competition in Melbourne on July 6. To win the event Leutele will have to win three fights which are scheduled for three rounds.
"I'm quite excited as this will be my first fight since March. I'm ready to fight now so the extra three weeks is a bonus," Leutele said.
"I've been training every day since the start of the year and at the weekend I went to Auckland for 10 rounds of sparring at Shane Cameron's gym.
"Obviously, if you lose in Melbourne you're finished for the night. If I win three fights there it will open more doors up for me at international level. I want to be the New Zealand heavyweight champion by the end of the year and if anyone wants to help with sponsorship I would love to hear from them," Leutele said.
The 2011 national youth super-heavyweight champion, who pounds the streets as a refuse collector during the day, scored a third-round TKO against Canterbury heavyweight Ali Loto in March.
That was his second professional fight. Loto was 10kg heavier than Leutele and had won three of his previous four pro fights.
Aussie-based former Manawatu fighter Kyle Brumby will also be on the card in Melbourne.
Langdon had planned on taking Leutele to Aussie earlier this year but Leutele's passport didn't arrive in time.