The first goal came early in the first half when O'Sullivan dispossessed the Hukanui Rototuna goalkeeper before placing the ball into an empty net.
Ngongotaha held onto their lead and went into the sheds 1-0 up.
Former All White and Manchester United trialist Michael Groom, who is head coach at Hukanui Rototuna FC, would have stressed to his team not to give away any more soft goals.
However, it was another error which led to Ngongotaha's second goal after the break.
O'Sullivan dispossessed a defender and beat the goalkeeper once again to make it 2-0.
Their third goal came after Harlem Simiona crossed a ball to O'Sullivan, who knocked it in for his hat-trick. O'Donoghue completed the victory with a nice goal.
In Federation Two, Rotorua United had a good first up win against the Ngongotaha reserve team at Jesse Martin Park.
Captain Lawrie Scott scored a hat-trick while Shadley Petersen also got on the score sheet winning the match 4-1.
Ngongotaha managed to pull a goal back against United through the boot of Brendon Watts.
United will be looking to earn promotion back to the Waikato Bay of Plenty top grade for next season.
Waikato Bay of Plenty football results on Saturday:
Federation One: Ngongotaha 4 Hukanui Rototuna 0, Cambridge 1 Old Blues 1, Otumoetai 1 AFC Fury 2, Tauranga Boys' College 2 Matamata Swifts 1, Melville United 2 Waikato Unicol 3, Tauranga City 7 Taupo 0.
Federation Two: Ngongotaha 1 Rotorua United 4, Tauranga City 2 Katikati 4, Papamoa 3 Old Blues 0, Te Awamutu 1 Tokoroa 0, Cambridge 3 West Hamilton 2.