The defending champions later tore Agoro Sare apart after a commanding 23-5 second quarter performance before putting the game to sleep with a quarter left after another 15-8 show in the third quarter.
The Joseph Were coached side sealed the win with a 27-2 fourth quarter show to win 84-27.
"I told you we will defeat them heavily," said Onjiko coach Were.
"The boys played well today. Despite a physical start in the first quarter, we composed ourselves to shatter their dreams of winning game one. We look forward to our second match today," said the former Sigalame Boys head coach.
Onjiko will face off Nyanchwa from Kisii County today from 2pm. Nyanchwa, who lost their county title to Kisii School two weeks ago, will begin their games against Migori today from 8am.
At the same time, Usenge shocked hosts Kisii School 39-30 in another match.
In the girls' category, defending champions Asumbi Girls saw off Tuk Jowi 31-28 to get off to a positive start.