He seemed bemused after his commanding performance, saying he had been concentrating more on his country's team prospects rather than the Individual competition.
Oiwa said: 'In training yesterday it didn't go very well, but today she was very good. When there was lightning before my Dressage time she was afraid of it, but then she was fine.
'I didn't expect that I could be at the top of the Dressage but I did my best and the horse did very well. I was only thinking about team results and I wanted the best result ever for our team. And that's still the goal for me, I'm not thinking about the individual.'
Oiwa, who came 11th at Badminton in 2005 and is now based with German competitor Dirk Schrade, said his horse Noonday de Conde will try her best in the cross-country tomorrow, adding: 'She is very honest, and she is always trying her best for me. I'll be really trusting her tomorrow.'
In the Team rankings, favourites Germany will continue their march towards potential Olympic Eventing Team gold on Day 3.
The reigning European Team champions showcased their enviable strength-in-depth by taking charge after the Dressage phase.
And such is the German team's quality under Chris Bartle's coaching direction, that they could even afford to drop world champion Michael Jung's Dressage score of 40.60. The three best scores of five riders in each team will decide the medals after Tuesday's Jumping phase.
Despite a concerted challenge by Australia, Germany's magnificent consistency - four riders placed in the top eleven - means they lead the gold medal race.
'The team has been really strong,' Jung said, after Ingrid Klimke, Dirk Schrade and Sandra Auffarth produced the top three German scores.
Solid performances from three Britons - Zara Phillips, Kristina Cook and William Fox-Pitt - left Great Britain in third place.
At the end of today, Italian Stefano Brecciaroli finished second in the individual rankings, ahead of New Zealander Mark Todd.