2018 season

Geylang International signed Zainol for the 2018 season behind Jasper Chan. After Chan suffered an injury within his first few games. Zainol was given the nod in goal and started in Geylang's 17 league matches.

During a league match against his former club, the Warriors, at the Singapore National Stadium, Zainol made a string of saves to keep his former side at bay in a 2–0 victory, gaining him the nickname "King of Kallang".

2019 season

In the 2019 Singapore Cup 3rd place play-offs against Brunei DPMM, Zainol made the winning save in the penalty shootout to help the Eagles clinch third place.

2020 season

During a league match against Lion City Sailors on 18 October 2020, Zainol was sent off after performing a reckless tackle on Arshad Shamim, conceding a penalty at the same time which back-up goalkeeper Hairul Syirhan could not keep out.

Zainol, along with Geylang International, qualified for the 2021 AFC Cup group stage which was promptly cancelled in the following season.

2021 season

Zainol started in goal against Lion City Sailors on 11 April 2021 at Our Tampines Hub in an 8–0 defeat to the hands of the visitors.

Zainol started only 6 out of Geylang's 14 matches at the time and declared retirement from professional football on 17 August 2021 due to the logistics behind professionalism in Singapore football. Zainol citing that football wasn't a secured job for him due to contract negotiations and decided to sign on with the Singapore Civil Defence Force. Zainol was also one out of five goalkeepers Geylang deployed in their entire league campaign.

Return from retirement

During the 2022 Singapore Premier League season, Zainol was found wearing Hougang United's tees in the stands over the course of the season.

On 18 June 2022, Hougang United announced that Zainol had come back from retirement and signed for the Cheetahs. He was a used substitute in their 8–0 defeat against Johor Darul Ta'zim in a friendly. Zainol made his return to the league on 15 October 2022 against the Young Lions, winning 5–3. He ended his league campaign with a clean sheet against Balestier Khalsa before being substituted on for Aizil Yazid in the 81st minute.

Zainol started in all but one game in Hougang's Singapore Cup run, helping the Cheetahs to their first ever silverware.

2023 season

Following the arrival of Zaiful Nizam from Geylang International, Zainol was yet again relegated back to the bench but still made appearances for the club's Under-21 Center of Excellence squad. He would then make his first start of the season against Balestier Khalsa in the second round after Zaiful suffered an injury. Zainol produced seven saves in a 3–1 loss, playing a near perfect game as he stopped several shots from close range. He made his second consecutive start of the season in a 1–0 loss against Tampines Rovers.