Nichols' attorneys questioned 96 witnesses over 11 days in a case that was shortened when Judge Steven Taylor limited the defense's ability to offer evidence of alternative suspects in the bombing, which killed 168 people.
Prosecutors rested their case April 30 after questioning 151 witnesses over 29 days. They planned to question at least a dozen more witnesses to rebut defense testimony.
Closing arguments are tentatively scheduled to begin Monday afternoon.
Nichols, 49, is serving a life prison sentence after a federal jury in 1997 convicted him of conspiracy and involuntary manslaughter in the deaths of eight federal law enforcement agents in the April 19, 1995, bombing of the Oklahoma City federal building.
In Oklahoma, Nichols is charged with 161 counts of first-degree murder for the deaths of the other 160 victims and one victim's fetus.
Defense attorneys allege that other coconspirators gave executed bomber Timothy McVeigh substantial help in planning the bombing of the Alfred P. Murrah Federal Building.
Taylor ruled on April 21 there was no substance to defense allegations that McVeigh had links to a gang of white supremacist bank robbers and residents of Elohim City, a separatist enclave in eastern Oklahoma.
By Tim Talley