Integrating quantum key distribution (QKD) into optical networks is a crucial step toward the adoption of quantum technologies in existing telecommunication fiber infrastructures. However, state-of-the-art solutions face significant challenges, including sensitivity to classical noise, particularly spontaneous Raman scattering, limited transmission distances, and varying network conditions. In this work, we present a novel discrete-variable QKD system running a time-bin BB84 protocol operating in the O-band (1295.56 nm), where the impact of dominant noise sources is effectively reduced. Combined with passive narrow-spectral filtering at the receiver, our system demonstrates robustness across diverse dense wavelength-division multiplexing scenarios, making it practical for real-world deployments. We validate its performance through extensive testing, showing stable key generation coexisting with different classical traffic conditions up to 17 dBm of total launch power. These results represent a significant advancement toward the integration of QKD into existing fiber networks, paving the way for secure quantum communication on a large scale.