For the availability, we would suggest you walk into the nearest dealership as they will be the better person to assist you because it depends on their stock book. You can click on the following link to see the details of the nearest dealership and selecting your city accordingly - Car Showrooms
The difference between Tata Nexon XZ plus S and XZ Plus O is that Tata Nexon XZ plus O comes with the Remote Trunk Opener, Trunk Light, Rear Reading Lamp, etc. which is not there in XZ Plus S variant. Moreover, you can click on the link to see the complete comparison.
Apart from exterior dual tone colour, there is no different betweem XZ Plus Diesel S and XZ Plus DualTone Roof Diesel s. they come with touchscreen infotainment system by Harman Kardon with Android Auto and Apple CarPlay.
Tata Nexon is offered with either a BS6-compliant 1.2-litre turbocharged petrol engine or a BS6-compliant 1.5-litre turbocharged diesel engines and offers both these engines with either a 6-speed manual transmission or a 6-speed AMT.