We present an off-the-shelf, low-latency Optical See-through Head-Mounted Displays (OST-HMD) for Augmented Reality (AR). Temporally consistent visualization is crucial for realizing immersive AR experiences. This is challenging since it requires both accurate head-tracking and low-latency rendering of AR content. Building a system which meets both constraints usually requires experts on computer vision/graphics and expensive display hardware. This work demonstrates that such high spatio-temporal fidelity is achievable with commodity hardware available today. We build a custom OST-HMD system that consists of a virtual reality HMD, i.e., the Oculus Rift DK2, and half-mirror optics, and adapt the rendering pipeline in order to integrate the OST-HMD calibration framework. An evaluation with a user-perspective camera shows that the system achieves mean temporal error of <1 ms (95% reduction of the latency from naive, no-predictive rendering), and median spatial error <0.3° in the viewing angle with maximum error at most 1.0°.