Han Gwang-Ok, a former aide to late President Kim Dae-Jung, was named chief of staff, an appointment the presidential Blue House said would help get the rattled administration back on track.
The 74-year-old veteran politician, a long serving liberal who switched sides and joined Park's administration in 2013, has dedicated his life to "democracy and national reconciliation".
"He is well suited to assist the president from the citizens' perspective," presidential spokesman Jung Youn-Kuk said.
In an effort to deflect some of the criticism, Park had been urged to create a neutral cabinet by bringing in members from outside her ruling conservative Saenuri Party.
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She has reached across the traditional political divide with a host of new appointments, including tapping a liberal politician as her new prime minister - a largely symbolic post.
But the opposition has dismissed the reshuffle as a smokescreen, calling for a full investigation of Park's relationship with Choi and vowing to block the new prime minister's nomination by wielding their combined parliamentary majority.
Prosecutors have been questioning Choi, who is being held in emergency detention, and have asked for a court warrant to arrest her on charges of fraud and abuse of power.
Choi has denied allegations that she used her friendship with Park to strongarm companies like Samsung into donating large sums to non-profit foundations that she then used for personal gains, Yonhap said.
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