The NYSE McClellan Oscillator is a technical indicator and it serves best as a simple measure of when the market goes from bullish to bearish. When its value crosses below the zero line as it did yesterday, the big players in the market turn bearish shown by today's big gap down.
This indicator alone cannot be used to determine a market top but when used in conjunction with the Percent of S&P stocks above 50 day moving average and the S&P's current GAAP PE and whatever the leading sector is for the market which is currently oil leading the market downward then you have the four components necessary to be conclusive that a market top has been reached. These other three components are shown in the charts below.
Trade well my friends