The vote count in Tain could decide the final outcome as John Atta Mills, the opposition leader, leads ruling party rival Nana Akufo-Addo by only around 23,000 votes out of more than 9 million votes cast.
Tain has about 53,000 registered voters, thus giving it a decisive role.
The Tain constituency was the last of 230 nationwide to vote. Polling in the constituency was halted on Sunday because of problems with ballot distribution.
"They have finished the election in Tain and they are beginning to count the votes," Theodore Dzeble, a spokesman for a local observer coalition CODEO, said.
Outgoing president
John Kufuor, the outgoing president of Ghana, who is stepping down after his maximum two terms in office, called on all sides to respect the outcome of the election.
Kufuor underlined the importance of meeting the constitutional timetable for the handover of power on January 7.
"I urge all stakeholders to yield to the authority of the electoral commissioner when he declares the results," Kufuor said.
"Any outstanding issues may be settled later by due process," he added, referring to claims and counter-claims of electoral irregularities.
Kufuor also appealed for calm until the results were announced.
Kwado Afari-Gyan, the electoral commission chief, said earlier he would release the results from Tain, a western district bordering Ivory Coast, as soon as they came in.
Officials said Afari-Gyan would likely announce the results Saturday.