When dissolved oxygen is high after filling at a beer factory, is the issue caused by sampling, transfer, or filling and sealing? Starting with measurement reliability, this article traces the process from the bright beer tank outlet to the filler inlet and sealed finished product. It explains key causes such as air ingress in pipelines, insufficient container purging, residual oxygen in the headspace, and sealing abnormalities. It also provides verification approaches for startup, restart, and increased-speed operating conditions, helping you reduce blind adjustments, identify the source of oxygen pickup more efficiently, and protect beer flavor and packaging quality.