The cooling off in global oil prices led to a positive start to trade on Wednesday on both the bourses. But it did not last long. After the initial show of strength, National Stock Exchange’s main index Nifty 50 slipped and went into the red territory the first hour. But a sharp rise in the stocks of oil marketing companies, with some gaining between two and four per cent brought the index back into the green area. Helping the index along the way was the rise in large banking stocks.