* OPEC sees increased demand for its oil in 2015 * OPEC also slashes forecast for supply outside its group * Some traders still bearish, Citi says WTI could fall to even $20 * S&P puts negative outlook on Saudi Arabia after oil price drop (Recasts with settlement in Brent and U.S. crude, adds S&P's negative outlook for Saudi Arabia, paragraphs 1, 4-7) By Barani Krishnan NEW YORK, Feb 9 (Reuters) - Oil jumped for a third straight session on Monday as OPEC forecast greater demand for crude this year than previously thought and projected less supply from countries outside the producer group. The Organization of the Petroleum Exporting Countries forecast that demand for OPEC oil will average 29.21 million barrels per day (bpd) in 2015, up 430,000 bpd from its previous forecast. The group also slashed its outlook for crude supply growth in **n-OPEC countries. Oil prices have been trying to find a floor after a brutal selloff that wiped out over half of the market's value since June.