Two power-packed bursts by the Irish in the opening minutes yielded crucial tries against the Cherry Blossoms, and O'Donovan was evidently pleased with the seven-try performance, played out in difficult conditions.
O'Donovan said: "The aim of the tour was to get two wins so we are happy and satisfied. In the first half the humidity didn't make it easy on our players - they were sweating a lot and tired as the game went on.
"We tried to make the most of our fresh players in the second half, unloaded the bench and it was very pleasing to hit the Japanese for five tries after the break."
Irish captain David Humphreys, who leapfrogged Ronan O'Gara in the record points-scoring stakes - adding 17 points onto the previous mark of 525, which was shared by the fly-halves after the first Test, added: "We're obviously delighted. We tried to play a different style form last week and keep the ball in hand.
Japan captain Takuro Miuchi, who was again an integral figure for the home side at number 8, said: "In the first Test the lineout wasn't working well. We worked on it in training, but today the Irish players out-thought us and attacked from different angles."
Japan coach Mitsutake Hagimoto, who began working with two new French assistants Jean-Pierre Elissalde and Edmond Jorda in March, admitted: "The result says it all.
"It's hard to master the French style of rugby in just three months. But to score two tries from turnovers is a great thing for Japanese rugby.
"The players have come on a lot since losing heavily to Scotland and Wales last autumn and will be tougher, both mentally and physically, for games like this one."