Shark Tank star Kevin O'Leary, chairman of O'Shares ETF Investments, said volatility is back.

"Everything old is new again. We've got volatility back just like we used to have ten years ago," O'Leary said to TheStreet, adding that strong U.S. economic fundamentals bolster the argument for putting more money into mid-cap U.S. stocks.

O'Leary's (OUSM - Get Report) exchange traded fund tracks high quality Russell 2000 stocks with strong balance sheets. 

U.S. companies aren't as affected by trade issues as large multi-national companies are, according to O'Leary.