Newcastle edged their NSW rivals in a tense first half at Hunter Stadium but could not convert their chances as they went to the half-time break locked at 0-0.
Sydney snatched the lead against the run of play as Ognenovski muscled his way into the area to head home a Del Piero corner on the hour mark, before the Sydney skipper sealed the win from the spot in the 73rd minute.
The win lifts Sydney above the Central Coast Mariners and the Wellington Phoenix in a congested table while Newcastle are four points out of the top six and in danger of losing touch for a finals berth.
The Jets looked the most likely to score in the opening 20 minutes with Adam Taggart having a couple of half-chances before a Heskey header created space for James Virgili, who fired a shot narrowly wide of the far post.
Despite the home side's dominance Sydney almost pinched the opening goal on the half-hour mark when Del Piero capitalised on an errant back pass from Ruben Zadkovich and fed the ball to Ranko Despotovic, who forced a fine smothering save from Jets goalkeeper Mark Birighitti.
Sydney went close again five minutes later when a Del Peiro free-kick found Ogenovski at the back post but he could only balloon his header attempt into the arms of Birighitti.
Both sides continued the arm-wrestle in the midfield as they went to the interval locked at 0-0.
Newcastle continued to threaten after the break and they created the best opportunity of the match when Zadkovich and Taggart played a one-two for the skipper to launch a stinging right-footed attempt on goal that was well-saved in a diving effort from Sydney custodian Vedran Janjetovic.
Sydney snatched the opening goal on the hour mark when Del Piero swung a corner to the edge of the six-yard box, where Ognenovski sent his flying header in off the underside of the crossbar for a 1-0 lead.
The visitors extended their lead via Del Piero's penalty in the 73rd minute after the former Italian international was brought down in the box by a Taylor Regan challenge. Del Piero stepped up to the spot and sent Birighitti the wrong way to make it 2-0.
Newcastle continued to toil hard in attack with substitutes David Carney and Joel Griffiths creating some half chances while a classy Del Piero back-heel and a stinging left-foot strike from Ali Abbas saw Birighitti save well at his near post, Sydney finishing strongly to record their second win in succession.
Newcastle Jets 0
Sydney FC 2 (Ognenovski 60', Del Piero 73' pen)
Crowd: 12,449 @ Hunter Stadium
