The Sierra Leone People's Party said in a statement that it "noted with utter dismay" the poll results announced by the country's election commission.
Mr Koroma was sworn in for another term on Friday after provisional results from the November 17 election showed he had won with 58.7% of the vote. The SLPP candidate Julius Maada Bio was second with 37.4%.
In its statement the SLPP said the party had documented "systematic and widespread irregularities, malpractices and injustices" that occurred on polling day which undermined the credibility of the results.
The poll is only the third since the end of Sierra Leone's horrific 11-year-civil war. If the opposition is unwilling to accept the results, it could undermine stability.