Webb, the defending champion, had eight birdies and a bogey to take a three-stroke lead over Tania Elosegui of Spain and South Koreas Lee Chang-hee. Clare Queen of Scotland, Beatriz Recari of Spain and South Koreas Kim Hye-youn were each four shots off the pace.
The 34-year-old Webb, chasing her third straight Womens Australian Open title and fifth since 2000, was pleased with her strong start at the Metropolitan Golf Club.
Im not surprised because I know what Im capable of but I was really unsure on how the course would play today, the seven-time major winner said. Its one round of golf but Im definitely giving myself a few pats on the back because its a very good score around here whatever state my game is in.
Webb hasnt won a tournament since beating South Koreas Shin Ji-yai on the second hole of a playoff in Melbourne last year and is hoping to break her drought this week.
Its a nice little boost to confidence, said Webb, who birdied four of her first six holes. Hopefully I can carry it on for the next few days.
Oh, who hadnt picked up a golf club before moving to Australia from South Korea in 2004 and now plays off a 2 handicap, was 13 shots back after a 6-over 79.
But after having to miss school to compete in qualifying and the Open, Oh hadnt given up hopes of making the cut despite a harsh self-assessment of her opening round.
I was really nervous when I started and really nervous when I finished, said Oh, who turns 13 in May. I enjoyed it, but I did not play good golf.