Koroma secured re-election in the first round of voting last month but his rival Julius Maada Bio's opposition Sierra Leone People's Party (SLPP) had announced its boycott of all levels of government in protest, threatening to undermine an otherwise widely praised vote.
The opposition change of tack following the vote that is seen as a test for Sierra Leone's post-war recovery came after a meeting on Monday between the SLPP and the leadership of Koroma's All People's Congress party.
"It is a matter of fact, the oath of office has been taken and he's the president of the country," Banja Tejan-Sie, secretary general of the SLPP said on Tuesday.
Tejan-Sie said the boycott was likely to be lifted following a meeting of the SLPP's leadership, due within the next three days.
Koroma won just under 60% of the presidential vote on November 17, avoiding the need for a run-off, and his APC party gained eight seats to secure a majority with 67 of the 124 seats in Parliament. The SLPP took 42 seats, losing three.
Sierra Leone has seen a decade of post-conflict reconstruction, and the country, which has resources including gold, diamonds, oil and iron ore, has drawn billions of dollars in state revenues from mining and agriculture deals.
However, it remains one of the least developed nations on earth and the ruling party said the opposition move would allow the aid-dependent country to "move forward".
"It has brought a lot of relief to many Sierra Leoneans who were kind of tense because the main opposition leader had not met the president," Sheka Tarawalie, Sierra Leone's deputy minister of information, told Reuters.
"Now he has come out openly to acknowledge the president as elected leader of the country, the tension has gone away."
A 100-strong observer mission from the European Union said the advantages of incumbency meant there was no level playing field for the election but that it was generally free and fair. – Reuters