The bidding war for one of the last independent stock markets in northern Europe follows consolidation which has been driven by the need to spend on technology and new entrants.
By raising its offer to 158 Norwegian crowns per Oslo Bors share, hours before its opening gambit of 145 crowns was due to expire, Euronext outbid Nasdaq's 152 crowns a share.
Paris-based Euronext revealed its first offer in late December with the backing of slightly more than half of Oslo Bors shareholders, but did not win over the exchange's board, which then convinced Nasdaq to make a higher bid.
Although Euronext's Chief Executive Stephane Boujnah said its bid offered "clear and superior benefits", Oslo Bors Chief Executive Bente Landsnes told Reuters that she still backs Nasdaq's offer, which also has the support from the largest shareholder, Norwegian bank DNB.
"We've conducted a thorough evaluation of who would be the best owner, and we stand by that assessment. We have not had any further contact with Euronext following our board's endorsement of Nasdaq's bid," Landsnes added.
Euronext's Boujnah was on Monday in Oslo to drum up support for the deal, which already has the "irrevocable" support of 50.5 percent of shareholders in Oslo Bors, including a 5 percent stake owned by Euronext itself.
This means that for Nasdaq to win it either needs Norwegian regulators to back it over Euronext or for those who made commitments to Euronext's bid to let these lapse, which would be in August for some and in December for 38 percent.
"We want to make Oslo Bors a leader in the Nordic region. We want to grow Oslo Bors," Boujnah told reporters, addressing the concerns of some of the shareholders opposing Euronext's offer.
"We will maintain a very strong presence of Oslo Bors in the local (business) ecosystem, which is very important for them. And we can make Oslo Bors a leader in the Nordic region, which is also very important for them," he told Reuters.
Euronext has had only one meeting with DNB, on Jan. 17, and one meeting on Jan. 10 with pension fund KLP, which holds 10 percent of Oslo Bors and is also backing Nasdaq, Boujnah said.
"Let's quickly forget the past five weeks and let's focus on how we can develop the relationship between Oslo Bors, DNB and the Norwegian (business) ecosystem," he said.
Oslo Bors would diversify Euronext's revenue from shares and derivative trading, given Oslo Bors' leading position in seafood derivatives as well as oil services and shipping.
Euronext plans to appoint Oslo Bors's CEO to its managing board, with responsibility for all commodities operations. It said it would also invite "a leading figure from the Norwegian financial" to its board.
Originally, Euronext was invited to bid by a group of shareholders without informing the management of Oslo Bors, whose board reacted by saying it would seek new bidders.
Euronext, which runs exchanges in Paris, Brussels, Amsterdam, Lisbon and Dublin, is looking to expand but remaining opportunities are scarce as market operators either already belong to large groups or want to remain independent.
Large-scale mergers have also met opposition from competition regulators, who have blocked a planned tie-up between Deutsche Boerse and the London Stock Exchange.
(Reporting by Sudip Kar-Gupta, Inti Landauro, Terje Solsvick and Gwladys Fouche; Editing by Rashmi Aich and Alexander Smith)
By Inti Landauro and Terje Solsvik