According to a report by Ming-Chi Kuo of analyst firm KGI that was obtained by MacRumors, the iPhone 7 Plus dual-camera is equipped with optical image stabilization in the 28mm equiv. wide-angle lens, but not in its longer 56mm equiv. lens. In the report, KGI claims that optical image stabilization will likely make it into the tele lens with the next upgrade of the device in 2017 via a 'voice coil motor' upgrade:

“Note that for the dual-camera of iPhone 7 Plus, wide-angle CCM is equipped with optical image stabilization (OIS) VCM, while telephoto CCM only comes with general VCM. We believe the focus of the dual-camera upgrade will be equipping telephoto CCM with OIS CCM, so as to significantly enhance optical and digital zoom quality.”

In the same report KGI predicts that for the foreseeable future the dual-camera feature will remain exclusive to the larger iPhone Plus model which is expected to represent 30-40% of all iPhone sales. 

Apple limiting certain camera features to the larger model is not without precedent. In the iPhone 6 and 6s generations only the Plus devices were equipped with optical image stabilization. With the 7th generation this has now trickled down to the standard model and we would expect the same thing to happen with the dual-camera at some point in the future. 

During the launch presentation Apple did not mention a lack of OIS in the tele lens. In practical terms this would mean that, when shooting with the tele module,  the camera has to use higher ISO values in lower light to avoid camera shake. That said, we'll have to wait for a review model to find what this means for image quality.