A relieved Ormsby, who has led from the first round, birdied his last hole for a third-round two-over-par 74 as Singapore's Lam hit four bogeys in his last five holes on another windy day at the Delhi Golf Course.
"It was definitely a sign of relief when I birdied that hole. My putter was frustrating me a bit and it was overall a tough day," Ormsby, 33, told the Asian tour.
"I started fantastic but my putter was a bit dry today and I hope to get it going again tomorrow."
Ormsby started with a birdie in his opening hole but his game fell apart as he made four bogeys to make the turn on 39. He found his touch back on the back nine, carding eight pars before the birdie on the last.
"The pressure's always mounting, that's golf. It's just how you deal with it," he said. "I just wasn't making the putts. But you just got to keep going on this golf course and see where you finish."
India's Shiv Kapur (72) and Thailand's Boonchu Ruangkit (73), who is looking to become the oldest winner on the Asian Tour at 56, were tied third, three shots behind the leader.
Lam, who lost his Asian Tour card last year, had opened up a four-shot advantage after his birdie on the 11th hole before his game fell apart.
"I'm not going to dwell on what has happened in the last few holes. More importantly, I'm still in it and I look forward to tomorrow," Lam said.
"I definitely had some positives that I could take out of today and I need to work on my finishing as it hasn't been that great. I like the position where I'm in.
"Even though I missed a few putts coming in, the strokes were solid. It's probably more a misread than anything else."
Thai veteran Boonchu was already feeling the strain of three rounds under difficult conditions on the course.
"I need to go back and allow my body to recover before I go back and play my last 18 holes. It'll be nice if an old man like me gets a win tomorrow," he said.
"Course conditions have been tough the last three days and whoever wins tomorrow is definitely a deserving winner."