Food ordering with ONDC via Ola going to impact Zomoto Swiggy

Not really. When something is cheap, it's subsidized to get market share. But it gets devalued soon.

A general opinion, Ola is a shit company. From employees perspective it's worst.
Unfortunately Swiggy has also become same....and still continuously degrading. I'm not sure if we should even believe so called fake leaders form these companies in any aspects.

The problem with ondc is the delivery infrastructure that Zomato swiggy has in place in scale, there is a chicken egg problem here as without orders in volume delivery boys won't come and without delivery boys in large numbers it won't be able to increase delivery volumes

That's because ondc charges way less commissions and in swiggy/Zomato the menu prices are inflated to account for this.