CHICAGO--(BUSINESS WIRE)--Fitch Ratings has downgraded Oi S.A.'s (Oi) long-term foreign and local currency Issuer Default Ratings (IDR) to 'BB' from 'BB+'. Fitch has also downgraded Oi's National long-term rating and National long-term debentures to 'AA-(bra)' from 'AA(bra)'. The Rating Outlook on these ratings has been revised to Negative from Stable. Fitch has also downgraded Oi's senior unsecured and secured debt, and the senior unsecured notes issued by Oi Brasil Holdings Cooperatief U.A (O