Canada, ranked 15th in the world, rallied twice to earn a draw as the fifth ranked India finished the group phase with seven points from five matches.
The Dutchman was clearly not happy with the way India conceded two goals in 19 minutes with Scott Tupper scoring back to back penalty corners.
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"If we did that we would have been ahead. We kept Canada alive and gave the opportunity to draw, that's something we have to learn," Oltmans said after their match at Pitch 1 of Olympic Hockey Centre in Deodoro.
"The energy level dipped especially in the second quarter where so many chances to score goals were missed. That was the quarter we should have made the difference in my point of view."
India struck through Akashdeep Singh and Ramandeep Singh in the 33rd and 41st minutes respectively, while Tupper scored in the 33rd to help Canada draw level 1-1 and then in the 52nd minute, both times converting the penalty corners, to share honours.
Oltmans said, "We took the advantage but could not make it happen. We conceded penalty corner and a goal again. They should have been aware of such situations."
Oltmans said the team became complacent and over excited after scoring two goals.
"You should not get over excited after scoring a goal. You should just stick to your structure."
On their quarterfinal prospects, the coach said: "A quarterfinal is always a strong game. There's no doubt about it. We played Belgium a couple of times in the near past. World Hockey League (0-1 in December) and the Champions Trophy (1-2 in June this year).
Read our full coverage on the 2016 Rio Olympics
"Both times we lost by one goal difference. All are strong sides. Let's see what we can manage here. It will be an exciting match."
India's regular captain PR Sreejesh said they would have to pull up their socks before the big match.
"The energy level was a bit low, we conceded from penalty corners and some poor goals. We got opportunities and could not convert them.
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