Osotspa's stunning performance was the highlight of the weekend's action, with Ivorian Anthony Moura-Komenan their star man.
Moura-Komenan scored a hat-trick in the first hour, following up goals in the third and 16th minute with his treble just five minutes into the second half.
There was still time for Aron da Silva and Hiromichi Katano to net late goals also, as Osotspa's rout was wrapped up in style.
South Korea forward Joo Sung-Hwan scored the game's only goal in the 17th minute as Singhtarua – who won promotion to the top flight last term – continued their bright start.
Chatree Chimtalay put Bangkok in front in the first half but Teeratep Winothai's red card swung the match in Muang Thong's favour.
Chonburi remain unbeaten this season after they beat Chiangrai United 2-1 on Sunday – Thiago Cunha scoring both goals for the victors.
