While with the firm, Koo, Lee Fu-tien, and four other Taiwanese lawyers served as liaisons between taishang based in mainland China and the businesspeople's Chinese attorneys.

[6][7] In 2014, he joined the defense team of Chiou Ho-shun,[8] a man subject to the longest criminal case in Taiwanese judicial history who had been imprisoned for the murder of Lu Cheng in 1987.

[9][10] In December 2015, Koo, representing the Democratic Progressive Party as a whole, charged Kuomintang chairman Eric Chu with attempting to buy votes.

[20] Koo also coauthored amendments to the Act Governing Relations with Hong Kong and Macau in an attempt to simplify the process for political asylum-seekers from those areas to Taiwan.

[21] He also proposed an amendment to the Narcotics Hazard Prevention Act, stating that people involved in the illegal drug trade should be treated for addiction prior to being put on trial.

[22] The Act Governing the Handling of Ill-gotten Properties by Political Parties and Their Affiliate Organizations, which he helped to write, was passed in July and Koo was named to a commission set up to investigate questionable assets in August.

[28][29] Koo was appointed to the chairmanship of the Financial Supervisory Commission in September 2017, succeeding Lee Ruey-tsang on the same day that William Lai replaced Lin Chuan as premier.

The blacklist bans them from entering mainland China and the Special Administrative Regions of Hong Kong and Macau, and restricts them from working with Chinese officials.