Ian Madigan knocked over three penalties but the Saints ran out 18-9 winners, with a Jamie Elliott try at the death also denying the Irish province a bonus point.
Leinster are still in pole position in Pool 1 - four points ahead of Northampton - but O'Connor believes his team's lack of precision handed the English outfit an opening.
"We were inaccurate," he said. "We didn't look after the ball as well as we would have liked. We certainly didn't look after it as well as we did last week. That let Northampton into the game.
"They were playing for pride for a large part of it. We understood that. We knew they would come back at us and I think the defence at the end (from them) summarised the massive effort throughout the game.
"They kicked it into the corners and drove and made it hard for us to get any field position in the game and off the back of our ill-discipline and penalty count they had opportunities to do that.
"That was our fault. We didn't control possession and didn't get field position and we probably didn't put them under enough pressure.
"We talked a lot about that during the week but there wasn't a complacent edge to us. We were just inaccurate off the back off their commitment."
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