In sharp contrast to the ruling party's crowded presidential race, Mr Ozawa, one of the founders of the Democratic Party of Japan, was unopposed.
The convention was largely a formality - the party had already reappointed him as its leader on Sept 8. Sunday's vote was held to finalise that and give Mr Ozawa a chance to publicise his policies.
Though he doesn't have the forces behind him to take over Mr Fukuda's post, Mr Ozawa's re-election was seen by many as a sign the political gridlock that has largely paralysed Japan's political world for the past two years will continue, if not worsen.
Under his leadership, the Democratic Party of Japan has been making big gains against the ruling Liberal Democrats, and took control of the upper house of parliament in elections in July last year.
It has since managed to block or stall many of the main policy initiatives of the ruling party, and helped force out Mr Fukuda and his predecessor, Shinzo Abe, in a span of less than two years.
Mr Ozawa is now calling strongly for snap elections in the powerful lower house to test the ruling party's mandate with the voters.
Polls indicate such calls have public support and the rising pressure could force the Liberal Democrats to comply as early as next month.
The Liberal Democrats' inability to override the Democrats' stubborn resistance has led to what many in the party have called a crisis of both leadership and direction.
The abrupt resignation of Mr Fukuda earlier this month - he said he felt he could not deal with the opposition's uncooperative strategy - forced the Liberal Democrats to call elections for the party presidency, and Sunday was the final day of campaigning.
The elections will be held Monday. The winner is virtually assured of becoming prime minister when parliament opens on Wednesday because the Liberal Democrats and their junior coalition partner hold a majority in the lower house, which makes the final decision. -- AP