The capacity crowd at the Kashima Soccer Stadium had endured a nervous first-half as Islamic Republic of Iran's Persepolis took the match to the home team, with the Central Asians unlucky not to have taken the lead on a couple of occasions.
A more settled Kashima were able to control the match better in the second half and Brazilian duo Leo Silva and Serginho struck one each for the Japanese side to take a 2-0 lead into next Saturday's return leg at Tehran's Azadi Stadium.
"I stressed to the players during the break that we had not played to our normal standard from the first blow of the whistle in the first half," said Oiwa.
"Persepolis were very positive and aggressive while we were slightly negative in our play."
Persepolis certainly started brightly as they pinned Kashima in their own half and the Iranians had more looks on goal than the Japanese side did.
Oiwa said he told his players to play a more attacking game in the second half.
"We were too defensive in the first half and this allowed Persepolis to come at us. We changed that in the second half.
"We pushed forward more, used our flanks more freely which stretched their defence. This allowed us to go through the middle and create chances and score."
Oiwa described his team's second half performance as ideal but said Kashima can't yet think that they have their hands on a first ever AFC Champions League title.
"This was just the first half and we need to focus on the second leg and our J League challenge as well. It was good to have won but we must concentrate on getting ready for the second leg."
