The loss ended Allen’s hopes of winning his home event for the third time in four years and he was magnanimous in defeat, conceding that his opponent deserved to win the tense deciding frame.
“I know I had a half chance at a pink in the middle part of the frame but the way the balls were, Ollie was always in control.
“His safety was very, very good and I knew I was always going to have to fight for everything. I didn’t play well enough, Ollie deserved it.
“Some days it just doesn’t work. That’s all I’m going to put it down to. I’m not going to over-think it.”
Also on Wednesday, four times world champion John Higgins went down 4-2 to Barry Hawkins, while 2023 world champion Luca Brecel bowed out 4-1 to China’s Pang Junxu.
Defending world champion Kyren Wilson survived a scare when he came from 2-1 down to edge out Anthony McGill 4-3 in a compelling and mostly high quality contest.
McGill posted two century breaks while Wilson put together runs of 77, 62 and 70.
A break of 86 ensured Hawkins took the opening frame and the world number 19 went on to lead 3-0.
He appeared to be on his way to a straightforward victory but missed a simple yellow with the remainder of the colours at his mercy.
Higgins cleared up to reduce his deficit, then clawed his way back to trail 3-2 and compiled a break of 70 in the sixth.
Hawkins edged across the line however to book a place against world number one Judd Trump in the last 16.
Last year’s NI Open runner-up Chris Wakelin lost 4-1 to Ma Hailong while English Open champion Neil Robertson eased past Wang Yuchen 4-0.
Lei Peifan saw off Tom Ford 4-1, Elliot Slessor defeated Long Zehuang 4-2 and Jimmy White lost 4-2 to Martin O’Donnell.