Digital impressions of teeth, obtained through intra-oral scanning, allow for more efficient and cost effective treatments of many dental indications. Current state-of-the-art intra-oral impression acquisition systems make use of a separate monitor to show the scanning progress, forcing the dentist to divert attention away from the scanner and the patient. In this paper, we present an augmented reality based solution to this problem. During the scanning process, an optical see-through head-mounted display is used to show an online overlay of the dynamic dental model onto the patient's teeth. The dentist can then fully focus on the patient and the scanner, while still being able to keep track of the current state of the model. This type of novel application, which fundamentally changes the human-computer interaction of intra-oral scanning systems, requires a fast and accurate registration of a dynamically growing model onto a glossy, partially occluded surface at a very small scale. To meet this demand, we propose application tailored algorithms for indirect high accuracy online 3D teeth tracking and optical see-through head-mounted display calibration. Experimental results indicate that our system does have a potential to noticeably facilitate intra-oral scanning in the future.