Is it mandatory to get PIS letter on NRO account?

No, as per SEBI rules, an NRI can invest in Indian stock market through channelizing their investment via PIS or non-PIS route. If an NRI use NRO bank account for investment then obtaining a PIS permission letter is no more mandatory as an NRI can use non-PIS NRO account to invest in Indian companies and trade in F&O segment.