Oppenheimer Asset Management's John Stoltzfus believes too many investors are watching the technology and small-cap rallies like a "Vegas lounge act."

In an interview on CNBC's "Trading Nation," the firm's chief investment strategist suggested it's a big mistake to view those groups as a sideshow to the struggling broader market. Rather, he suggests, it's the bull market's main event right now.

"There's such focus on large cap, and Dow 30 stocks that often times investors forget about the historical outperformance that could be provided from owning mid-cap and small-cap stocks," Stoltzfus said Monday. "Investors still have good reason to be positive on equities."

The Russell 2000 index of small caps hit a fresh record high Monday. It's now up nearly 8 percent this year while the Dow, which got hit with two corrections this year, is virtually flat.

According to Stoltzfus, the tech-heavy Nasdaq, which closed at a record high on Monday and has rallied more than 10 percent this year, also signals the bull market is firmly in place.

"It is right in the trend of where we are experiencing growth — not only in the U.S. but around the world," he added.

Stoltzfus acknowledged that geopolitical risks such as U.S.-China trade tensions could continue to bruise the broader markets in the near term. But he's sticking with his S&P 500 year-end target of 3,000, a nearly 9 percent gain from current levels.

"We would expect that if we get a positive resolution into these trade negotiations, even if it's just progress and not perfection," Stoltzfus said. "We think the markets could rally much higher than where they have rallied to so far."

He urges investors to be well diversified and nimble — giving technology, consumer discretionary, industrials and financials "outperform" ratings.