'Troublesome' Thompson - he embraces that nickname - has climbed the rankings since turning pro in 2015, earning 12 knockouts. In his last outing, he stopped domestic rival Luke Watkins.
But a shot at Opetaia, who is unbeaten in 22 bouts, came sooner than Thompson or his team expected.
Poland's mandatory challenger Mateusz Masternak withdrew from negotiations, Briton Richard Riakporhe withdrew from the purse bids and former champion Mairis Briedis, next in line, was ruled out through injury.
"It was a tiny bit of a shock, but I've worked my way up the rankings," Thompson says.
"I've won the IBF international belt, European belt. Whatever happened with the other boxers passing up the opportunity, I was next on the list.
"There are a few people out there saying I'm not deserving of it, but I've done what I've need to do, won my fights. I feel like the shot has been earned."
Opetaia, making a first defence of the title he won again Briedis last year, is a strong favourite.
"Nothing great comes from comfort zones," Thompson adds. "To go in there as an underdog, that means a lot to me.
"I've got a load of people to prove wrong, but I also have a lot of people who have stuck by me, so I have those to prove right."
Thompson feels a win over Opetaia can lead to further, more lucrative, contests - perhaps an all-British unification fight against WBO champion Chris Billam-Smith.
"This is the fight where I can secure that belt and lock in those real big fights, against those big names, domestically or world scene," Thompson says.
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