Playing in his 32nd World Championship - a record he shares with Steve Davis - the seven-time world champion showed flashes of his enduring brilliance despite a lengthy absence from the tour.
O'Sullivan, 49, had not played a professional match since January, when a frustrating run of form culminated in him snapping his cue and discarding it after four defeats in five matches at the Championship League.
That dramatic moment was followed by a spell away from competition, during which O’Sullivan withdrew from several major events, including the prestigious Masters, the Welsh Open, and the Grand Prix.
He cited the need to focus on his health and wellbeing, later stating that simply returning to the Crucible was "an achievement in itself" amid ongoing mental health challenges.
Against Carter, O’Sullivan initially looked rusty, losing the opening frame. However, he quickly settled into rhythm, producing a superb break of 107 - his 1,283rd century in professional play and his 208th at the Crucible, both extending records that may never be touched.
That effort helped him establish a 3-1 lead.
He appeared poised to extend his advantage further with another century, only to miss a straightforward red at 85.
A chance to close the session with a two-frame cushion slipped away when, at 46-5 up in the ninth frame, O’Sullivan missed a long red. Carter pounced to snatch the frame and limit the damage heading into Wednesday afternoon’s concluding session (15:30 CET).
O'Sullivan is seeking to break new ground by winning an eighth world title - something no player has achieved in the modern era.
His current haul of seven world crowns ties him with Stephen Hendry, whose 1990s dominance once seemed untouchable.
Should O’Sullivan lift the trophy again this year, he would move clear as the most decorated player in Crucible history.
Murphy on the rise
Elsewhere, former world champion Shaun Murphy produced a clinical display against Crucible debutant Daniel Wells, racing into a commanding 7-2 lead after their opening session.
Murphy, who famously won his first world title 20 years ago in 2005, looked every inch a serious contender again, firing breaks of 63, 100, 57, 138, and 76 to take control of the match.
Wells, who fought through three rounds of qualifying to reach the main stage, briefly shone with centuries of 131 and 109, but errors - including a missed black in the final frame - allowed Murphy to punish him ruthlessly with an 89 break.
Murphy needs just three more frames when they resume on Wednesday morning (11:00 CET).
After claiming the Masters title in January, Murphy is bidding to become the first player since Judd Trump in 2018/19 to win both the Masters and World Championship in the same season.
Speaking of Trump, the world number one began his Crucible campaign on Tuesday evening, facing a tricky opening clash against China's Zhou Yuelong. At the close of the day, Trump is leading 6-3.
Trump, who lifted the title in 2019, comes in as the bookmakers’ favourite but will know the dangers of a sharp opponent early on.
In other results, China's Ding Junhui booked his place in the last 16 with a 10-7 victory over Zak Surety.
Ding, a three-time finalist at the Crucible, held a 6–2 advantage overnight but had to weather a flurry of big breaks from Surety, who compiled four centuries - 104, 109, 136, and 110 - across five frames in a spirited fightback.
Meanwhile, Chinese 10th seed Zhang Anda leads compatriot Pang Junxu 5–3 after their opening session, although their match will be completed on Wednesday morning after running slightly behind schedule.