The Qatar Investment Authority filed for regulatory approval to take a bigger stake in OMX on Tuesday, raising the prospect of a bidding war for the Nordic firm with rival Dubai and its ally Nasdaq.
The authority, which made the request through its Qatar Holding subsidiary, has previously said it controlled 9.98 percent of OMX shares, which is just below the 10 percent threshold at which the regulator must approve ownership.
Qatar started acquiring stock in the Nordic and Baltic exchange owner just after Borse Dubai and U.S. exchange Nasdaq announced they were teaming up to buy OMX.
The examination is an assessment of whether the new owner is suitable to have a significant influence in the company, Sweden's Financial Services Authority said in a statement.
The FSA requires firms to request approval for owning more than 10 percent in companies deemed strategically important. If a firm gets approval, it can own any amount of stock.
If you are fit and proper to own 10 percent, obviously you can't not be fit and proper to own 50 percent or 70 percent or 100 percent, said Helena Ostman, spokesman for the FSA.
OMX, as the owner of Sweden's stock exchange, is considered vital to the functioning of the local financial system.
State-run Borse Dubai last week said it owned 47.6 percent of OMX after it increased its cash offer to 265 Swedish crowns per share from 230 crowns, valuing the bid at about $4.9 billion, up from just under $4 billion.
Borse Dubai's chief executive Per Larsson later told Reuters he expected to cross the 50 percent barrier fairly soon.
Shares in OMX were up 2.4 percent at 282.50 crowns at 1115 GMT, indicating the market believes a counterbid by Qatar is possible.
OMX spokesman Jonas Rodny said the company had no information about the matter and was not aware of Qatar's intentions.
Both Qatar and Dubai want to create a financial hub in the Middle East. Nasdaq's goal is to gain a significant foothold in Europe to capitalize on upcoming regulatory changes which are likely to transform the exchange landscape in the region.
If it acquires OMX, Borse Dubai will transfer the firm to Nasdaq in exchange for 20 percent of the combined group as part of a complex tie-up. OMX's board has recommended the Borse Dubai offer.
The Swedish government owns 6.6 percent of OMX, but it has not said how it views the Borse Dubai/Nasdaq offer other than that it is an interesting proposal.
Financial Markets Minister Mats Odell has said the government will take a view in good time before the Borse Dubai bid concludes, which he expects could be around January.