Phase noise (PN) is a critical impairment at D-band frequencies (110 to 170 GHz), which are widely investigated as promising candidates for beyond 5G/6G ISAC systems. This paper evaluates OFDM based ISAC sensing performance under realistic oscillator impairments using a hardware-tuned 3GPP PN model at 130 GHz and FFT based radar processing. With a numerology of 480 kHz, results show that PN introduces range RMSE floors of 0.04 to 0.05 m and velocity RMSE floors of 0.12 to 0.18 m/s. Doppler sidelobe metrics also saturate, with PSLR around minus 6 dB and ISLR around minus 4 dB. These findings confirm that range accuracy remains bandwidth limited, while velocity estimation and sidelobe suppression are strongly PN-sensitive. The study highlights the importance of PN-aware waveform and numerology design for sub-THz ISAC and provides insights for future multi-band transceivers. Communication metrics and PN mitigation strategies such as PTRS and CPE tracking are left for future work.