The impeachment-threatened Park has been accused of letting long-time confidante Choi Soon-Sil meddle in a wide range of state affairs, and of ordering aides to leak confidential documents to Choi, who has no official title or security clearance.
The president is also suspected of colluding with Choi in strong-arming major conglomerates into donating funds to two dubious foundations Choi controlled.
The scandal has sparked mass rallies across South Korea, with hundreds of thousands of protesters calling Park a "puppet" of the Choi family and demanding she resign immediately.
A major parliamentary hearing on the scandal has summoned the country's most powerful business tycoons as well as Choi's close associates, including a prominent K-pop video director and a fashion designer.
Cha Eun-Taek, who once made videos for "Gangnam Style" star Psy, said Choi had once asked him to recommend potential candidates for culture minister and for a presidential adviser on culture.
The names he put forward to Choi, including one of his uncles, ended up getting the jobs.
"So I thought...'What kind of person could possibly do something like this?'" Cha told lawmakers, describing Choi's ties with Park as "extremely close".
Cha also said Choi had often sought his written opinions on cultural issues. These were later included word-for-word in some of Park's speeches.
Choi, whose influence over Park has led to her being called "Korea's Rasputin", has refused to testify to the investigative parliamentary committee despite warnings from lawmakers that she could be jailed for contempt.
Another member of her inner circle, a young designer named Koh Young-Tae whose relationship with Choi has been the subject of lurid media gossip, testified to the power Choi appeared to wield over senior administration officials.
He particularly noted her treatment of Kim Chong, a former vice sports minister who is now under arrest on charges of awarding lucrative contracts to Choi and trying to help her meddle in preparations for the 2018 Pyeongchang Winter Olympics.
"Choi constantly gave orders to Kim... as if he was her personal assistant," Koh told the televised hearing.
He also suggested Choi had controlled the president's wardrobe choices, saying he provided Choi with about 100 items of clothing and up to 40 bags that he saw Park wearing in public for years.
At the start of Wednesday's hearing, the chair of the parliamentary committee issued orders compelling Choi and members of her family to appear or face possible jail for contempt.
Choi had been slated to answer questions on Wednesday but refused the summons, citing health reasons and the need to prepare for her pending trial.
But Choi's niece, seen as her close aide, made an appearance and was grilled for allegedly using Choi's power to squeeze money from Samsung.
Samsung is accused of giving 1.6 billion won ($1.4 million) to a dubious sports foundation controlled by the niece, Jang Si-Ho, who was arrested last month for allegedly embezzling some of the funds.
Jang denied all accusations against her, saying she had only followed orders from her powerful aunt to whom she could not say no.
Samsung was the single biggest donor to Choi's foundations, and also sent Choi millions of euros to fund her daughter's equestrian training in Germany, in what critics describe as a bid to win policy favours.