It depends where the roses have been kept before you buy them - if they're under cover or inside, then they will need hardening off before leaving them outdoors all the time if your temperatures are still pretty cool, especially at night. But if they've been grown and kept outdoors, you can plant them up and put them out immediately - but don't keep them under cover until you do pot them up in larger pots, keep them outdoors.
Be aware though that Osoeasy roses are usually wider than they are tall, so they have a mounding growth habit, which means they'll likely tumble down over the edge of your planters. Review information of some of the varieties here http://www.homenursery.com/blog/bid/277574/Oso-Easy-Roses-Review
Grown in pots year round, it may be necessary to move the pots somewhere more sheltered in severe winter weather so that the potting soil inside does not freeze. These roses are hardy in your Zone, but plants are always more vulnerable in pots.