Only 65% of those allowed to vote have registered their intention to do so. That is a considerable drop from the last election three years ago, when about 90% of voters registered.
Turnout was reported to be strong early on Thursday.
More than 114,000 Omanis will elect a new Majlis al-Shura, an Islamic-style council that has no formal powers but is consulted by Oman's ruler, Sultan Qaboos, on new laws and public policy.
Only 65% of those allowed to vote have registered their intention to do so. That is a considerable drop from the last election three years ago, when about 90% of voters registered.
The authorities have more than tripled the franchise, but interest appears to have waned. The eligible voters were appointed by tribal leaders.
Free vote
The Omani Interior Ministry said it could not force people to vote, and that this was a free election.
It said it was not looking for the 99.9% result common among many fledgling Arab democracies.
The polls will close at 1900 local time (1500 GMT) and the results are not expected before Friday.
Role of women
In the council's last term, two women won seats for the capital Muscat - the first time women have stood in elections in the Gulf.
This time there are 21 women candidates among a field of 556.
Women in Oman, as in the rest of the Gulf, are veiled and largely segregated from men, except in their homes.
The BBC's Julia Wheeler in Dubai says Oman is nudging its people towards an interest in representative government, but the process is slow.
The Shura was founded in 1990, with the authorities citing the traditions and principles of Islam as set out in the chapter of the Koran called Surat al-Shura.
As well as broadening the voter base, the results from the 59 constituencies at this election will be considered final.
In the past, Sultan Qaboos had the final say over which candidates eventually sat in the chamber, regardless of how many votes they received.
Only about 600,000 of Oman's 1.6 million population are over 21 years of age, meaning that about 25% of the adult population is allowed to vote.
Party politics, political rallies and campaign advertising are all prohibited.