Walk.
Since you arrive from Extra Schengen and your destination is also Extra Schengen I don't believe there is passport control to go through because you are not entering Schengen, you are just in transit.
There will be security check (metal detector) before boarding to ORD and you will be asked to produce ID (passport) together with the boarding pass at boarding.
A transit visa is not required for citizens of India. A transit visa is required for the following countries:
http://www.esteri.it/mae/it/ministero/servizi/stranieri/ingressosoggiornoinitalia/visto_ingresso/paesi_soggetti_visto_transito_aeroportuale.html