Former Goldman Sachs Asset Management Chairman Jim O'Neill told CNBC on Thursday that there are signs that a 10-year bull market in stocks is set to end.

Speaking at the Ambrosetti Workshop on the shores of Lake Como, near Milan, O'Neill told CNBC's Steve Sedgwick that the U.S. market was "definitely not cheap" and "very sensitive" to any bad news.

Weekly jobless claims in the United States have fallen to their lowest level since 1969 and the former Goldman executive noted that it is entirely possible that the U.S. Federal Reserve could find itself quickly reversing a dovish outlook that has been in place since early 2019.

O'Neill said any suggestion that the Fed was tightening would upset both the U.S. stock and bond markets.

"The idea that maybe the markets were right all along and the Fed will be tightening by the end of the year is not a crazy idea in my view," he said, before adding, "We might be entering the end of the 10-year almost clear bull market in equities. The kind of volatility we have seen in the last six months is very different from the past 10 years."