Karun Nair (59) and debutant Abhishek Reeddy (54) shepherded Karnataka's innings with a 113-run fifth wicket stand but oce they departed, Karnataka witnessed a batting collapse with the hosts losing their last six batsmen for just 24 runs in the final session of the day. Opener Mayank Agarwal top-scored with a handy 68.
In reply, the Manoj Tiwary-led Rest of India side, who won a good toss on a green Chinnaswamy wicket, were 20 for one at stumps with Jiwanjot Singh (16*) and Paras Dogra (4*) at the crease.
Karnataka captain R Vinay Kumar, playing his 100th first class game, gave his team something to cheer about by removing opener Unmukt Chand on the first ball of the innings.
Aaron's lively spell of 6 for 63 gives Rest of India the advantage going into day two but their batsman will have to play out of their skin against a potent Karnataka attack.
With opener KL Rahul ruled out of the game with an injury, Agarwal and the prolific Robin Uthappa opened the innings for Karnataka.
But Aaron provided Rest of India with a crucial breakthrough early on, dismissing the dangerous Uthappa (10) caught behind in the eighth over.
Later on, he removed Agarwal in a similar fashion, leaving Karnataka at 107 for four. The opener tried to cut a delibery too close to his body, giving a simple catch to wicket-keeper Naman Ojha.
Nair and Reddy then came to Karnataka's rescue with a solid partnership. A high on confidence Nair after his triple ton in Ranji final looked in good touch and went on to collect 11 boundaries in his 99-ball effort.
But the duo's dismissal off successive balls proved to be the turning point as Aaron and Co ran through the lower-order.
Mumbai pacer Shardul Thakur (1/78) and left-arm spinner Pragyan Ojha (2/62) chipped in at the right time for the visiting outfit.
Incidentally, Rest of India wicketkeeper Naman Ojha took six catches, equalling the record of dismissals in Irani Cup history.
Brief scores:
Karnataka 244 in 77.1 overs (Mayank Agarwal 68, Abhishek Reddy 54, Karun Nair 59; Varun Aaron 6/63).
RoI 20/1 in 5 overs.
Image: Varun Aaron celebrates after picking up a wicket
Photograph: PTI