Euronext effectively won its bid for control of the Norwegian bourse last week after U.S operator Nasdaq threw in the towel.
After five months of battle and several counterbids, Norwegian regulators last month sided with Euronext, which runs exchanges in Paris, Brussels, Amsterdam, Lisbon and Dublin.
Euronext's success blocks Nasdaq's ambition of completing a sweep of the Nordic-Baltic region, where the U.S. firm already owns the stock markets of Sweden, Denmark, Finland and Iceland, as well as those of Estonia, Latvia and Lithuania.
On Tuesday, the pan-European market operator said it would keep the offer open until June 28 under the same conditions for Olso Bors shareholders who have kept their shares.
The Paris-based company offered 158 Norwegian crowns per share for Oslo Bors, valuing it at around 6.8 billion Norwegian crowns (616.07 million pounds).
Euronext's Chief Executive Stephane Boujnah said on Tuesday he expected the firm to close the acquisition by June 14. Euronext may seek to delist Oslo Bors if it secures more than 90% of its shares.
(Reporting by Inti Landauro; Editing by Michel Rose and Mathieu Rosemain)