[5] During a televised press conference in Hong Kong two days later, Koo swore at a man behind him, telling him to "move over, diu nei lo mo" (企開啲啦，屌你老母),[6] which surprised audiences and became a local meme.

[8] In 1998, Koo, along with Tsang Kin-shing, Leung Kwok-hung, and Lau Shan-ching, was fined HK$3,000 for disorder in public spaces over burning mock coffins during the visit by Jiang Zemin, paramount leader of China.

In December 2008, Koo was accused of attacking guards of Legislative Council during protests against public offering of Link REIT, a real estate investment trust, and was jailed for 7 days after choosing not to pay the HK$2,000 fine.

On 18 February 2013, Koo was jailed for 9 months for desecrating the national and Hong Kong flags in protest at human rights abuses in China and the alleged murder of Li Wangyang.

[14] On 28 January 2021, Koo received his tenth jail sentence of four months after desecrating the Chinese flag by writing "white terror" and "fascist horror" and flying it upside down, to voice out support with 15 democrats charged with illegal assembly.

[18] Koo was again arrested on 4 February 2022, reportedly under the security law for suspected incitement of subversion, before his planned demonstration outside the Hong Kong Liaison Office to protest China's Winter Olympics and call attention to political activists behind bars.

[19][20] Facing charge of attempting to commit a seditious act over a one-metre long coffin and a white flag with language including "down with the Chinese Communist Party" and "end one-party rule" found at his home, Koo was denied bail.