The specialist business and property lender will use the cash injection to launch in the United States before considering further overseas opportunities, Rishi Khosla, the firm's co-founder and chief executive, told Reuters on Friday.
The fundraising also represented a vote of confidence in British lending, despite mounting concerns over the potential impact of Brexit on the country's economy, Khosla said.
"Small businesses are the key growth driver in most economies and that's true in the UK," he said.
"The fundamentals are absolutely good over a period of time. If you tar a whole segment with the same brush I think that's incredibly unfair. You'll always have good businesses and bad businesses."
SoftBank's Vision Fund led the round with a $390 million investment, with Clermont Group providing the rest of the cash.
OakNorth, which also provides personal and business savings, has raised more than $1 billion since launching in September 2015 as investors show an appetite for financial technology firms to shake up the banking industry.
The latest investment values the company at around $2.8 billion.
SoftBank's technology-focused Vision Fund has multi-billion-dollar investments in U.S. companies, including WeWork and Uber.
"SoftBank always has a very large vision for businesses they get involved in and for us we have a relentless focus on small businesses," Khosla said.
"For us to fulfil that on a global basis having an investor who shares that vision is really important."
Khosla added that while the firm was now well-funded, there was a possibility it could float within five years.
British media had reported late last year that OakNorth had held talks with SoftBank about a potential investment.
In September, OakNorth had closed a $100 million funding round, which valued it at $2.3 billion at the time.
(Additional reporting by Philip George in Bengaluru; Editing by Alexandra Hudson and David Evans)
By Kanishka Singh and Iain Withers