Both the Dow Jones Industrial Average and S&P 500 ended at record highs Wednesday, despite evidence in the minutes from the Federal Reserve's latest meeting that showed the strongest dissent yet over the central bank's monthly purchases of $85 billion worth of Treasurys and mortgage-backed securities.
Since that mid-March Fed meeting, though, the government has released a report showing jobs growth much lower than expected last month.
(Read More: Jobless Claims Back on Track; Inflation Eases Off)
If the economy starts to stall, O'Neill said, market participants may believe that Fed policymakers won't take their foot off the quantitative-easing pedal after all. But he's not necessarily persuaded by the weakening-economy scenario.
"Before the data for March, it looked to me the U.S. economy was accelerating in line with what we've been thinking," O'Neill said. "I think it's quite hard to read aspects of that. [But] I'm not convinced the economy has slowed again."
—By CNBC's Matthew J. Belvedere; Follow him on Twitter