Oyama had six birdies and two bogeys but is just one stroke ahead of a crowd of 10 golfers at 3-under 69 with another nine at 2-under.
Defending champion Stacy Lewis is in the next group, which totals 16 players, at 1-under. That puts 35 players within three strokes of the lead with 36 holes to play.
Lewis, at No. 3 in the world rankings, is looking to make up ground on No. 1-ranked Inbee Park and second-ranked Suzann Pettersen, both of whom are skipping the tournament in Japan.
Winds affected play later in the day, affecting players including Lewis, who had birdies at Nos. 5 and 13 and a bogey at the 12th hole to go with 15 pars.
Oyama, a 13-year veteran of the JLPGA, said she has been fighting injuries recently, including a neck problem that has affected her game.
"This week I'm not actually aiming for the win though, I'm just looking forward in playing with the U.S. LPGA players and hopefully my neck won't get any worse than now," she said.
Oyama had a birdie at the second hole and moved in front with three straight birdies over Nos. 5-7 to make the turn at 4-under. She had second-nine birdies at 10 and 16 but also bogeys at Nos. 11 and 14 in between.