O.A.R.’s “Three Decades Tour” comes to The Amp June 19 — a day after kicking off the tour in Colorado Springs, and a day before playing to a sold-out Red Rocks crowd. “When you’re on tour, it’s really easy to get caught in the ‘go, go, go’ mindset,” said Richard On, songwriter and guitarist for O.A.R. “Of course, you’re trying to put on the best show possible, but sometimes you forget to smell the roses. This tour, we’re taking it slow, and we’l…