Oh, once viewed as a high-profile politician in the conservative bloc, has largely stayed away from the political scene after he stepped down as mayor in 2011 over a row over free school lunches.

He attempted to thwart the Seoul city council's bid to introduce free school lunches. But he later resigned, taking responsibility for his failure to block what he called a populist policy.

His return is sparking speculation that he will likely run in the party's race to pick a new chief in February 2019 and in the 2020 parliamentary election.

"I've come to join the party to contribute to forming the unity of the conservative bloc," Oh told reporters.

He vowed efforts to help the LKP win the parliamentary election while lashing out at the Moon Jae-in government.

"It will be desirable that next year's party convention should be a unity event that will involve participants from all factions sympathizing with conservative values," he said.

He is expected to assume leadership of the party's special panel on future vision and will be in charge of setting policy line that will differentiate from the Moon government's "income-driven growth" policy, according to party officials.

Oh, also a lawyer, entered politics in 2000 with a parliamentary election win. He was elected Seoul mayor in 2006 and reelected in 2010.

The public support rate for the LKP has been on the rise in recent weeks in reaction to public disappointment with the government's economic policy lapses amid signs of economic downturn.

But the party is also gripped by deepening internal strife over ways to reform itself ahead of the parliamentary election.

It suffered crushing defeats in the June local elections and parliamentary by-elections. (Yonhap)