Members of Odinga's Coalition for Reform and Democracy said the outgoing prime minister would file a petition a day later than had been expected, but they remained confident he will win the challenge.
Odinga has refused to accept the election results, which indicate Uhuru Kenyatta received 50.07 percent of the vote, Voice of America reported.
The election was the first nationwide since a 2007 election that was followed by weeks of violence and the deaths of 1,100 people, the report said.
The International Criminal Court indicted Kenyatta on charges of planning the violence. His trial is set to begin in July.