For the process monitoring during laser deep penetration welding, optical coherence tomography (OCT) has emerged as a promising method for the in-situ measuring of the keyhole depth or, as a result, of the weld penetration depth. Prior research on this topic with steel substrates has shown that reducing the ambient pressure significantly improves the detectability of keyhole bottoms. In addition to OCT measurements, high-speed recordings of the process zone enable the evaluation of the keyhole opening behavior in this study. Bead-on-plate welds in steel demonstrate that the reduction of the ambient pressure improves the measurements unambiguity and decreases the average keyhole diameter while the keyhole opening stability seems to remain mainly unchanged. Therefore, it can be concluded that the higher measurement unambiguity at lower ambient pressure is not caused by a higher keyhole accessibility of the OCT probe beam but rather by changed conditions inside the keyhole. Experiments with aluminum substrates validated the beneficial effects of the reduced ambient pressure on the detectability of the keyhole bottom.