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.
And that goal opened up the floodgates as Chayanan Pombuppha added a quick-fire brace.
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.
Singhtarua are top of the league after a hard-fought 1-0 win over Suphanburi.
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.
Ratchaburi are second after being held to a 0-0 draw at Sisaket on Sunday.
Muang Thong United returned to the winners list after overturning a deficit in a 2-1 win over Bangkok Glass.
Chatree Chimtalay put Bangkok in front in the first half but Teeratep Winothai's red card swung the match in Muang Thong's favour.
Kim Dong-Jin then levelled before one-time England international Jay Bothroyd bagged the winner just after the hour-mark.
Chonburi remain unbeaten this season after they beat Chiangrai United 2-1 on Sunday – Thiago Cunha scoring both goals for the victors.
Other results saw Air Force United draw 2-2 with PTT Rayong, while Police United edged Bangkok United 1-0 and Chainat beat TOT 2-0.
Rock-bottom Samut Songkhram drew 1-1 at Buriram United and BEC Tero Sasana won 1-0 at Army United.
