Israeli newspaper Haaretz reported on Thursday that Olmert testified that he had gone to war with a contingency plan he had approved four months earlier.
He reportedly told the commission on February 1 that he had asked military commanders in March 2006 if a contingency plan for military action existed in the event soldiers were captured along the Lebanon border.
Presented with several options, Olmert chose what Haaretz described as a "moderate plan" that included air strikes accompanied by a limited ground operation.
Yuval Steinitz, an opposition Likud party politician who was chairman of parliament's foreign affairs and defence committee until last May, said the newspaper report had astounded him.
"None of this ever happened," he told Israel Radio. "There was no intensive preparation for a possible imminent war."
Later on Thursday, senior Israeli officers criticised Olmert's comments, according to Haaretz, and questioned why the Israeli military was so poorly prepared for the conflict.
Ministers from Olmert's government coalition partners, the Labour party, also criticised the testimony during which the prime minister reportedly denied responsibility for the appointment of Amir Peretz as defence minister.
Olmert and Peretz have seen their popularity slump since the war, in which 158 Israelis, including 117 soldiers and 41 civilians, were killed. About 1,200 people were killed in Lebanon, including an estimated 270 Hezbollah fighters.
In response to the media reports, Elie Shaked, a spokesman for the Winograd Commission, told AFP news agency: "The commission does not comment on non-official information."
The commission heard from 70 senior politicians and officers during its investigation.
The only person to have accepted any responsibility for problems during the war, so far, has been General Dan Halutz, who resigned as army chief of staff in January without waiting for the commission's findings.
Halutz had been criticised for overestimating the effectiveness of air strikes during the war, while infantry and armoured units were poorly trained and equipped.
The Israeli military failed to achieve its declared goals of freeing the two soldiers and destroying Hezbollah's ability to fire rockets into Israel.