Open-Vocabulary Object Detection (OVOD) aims to localize and recognize objects from both known and novel categories. However, existing methods rely heavily on internal knowledge from Vision-Language Models (VLMs), restricting their generalization to unseen categories due to limited contextual understanding. To address this, we propose CODet, a plug-and-play framework that enhances OVOD by integrating object co-occurrence —-- a form of external contextual knowledge pervasive in real-world scenes. Specifically, CODet extracts visual co-occurrence patterns from images, aligns them with textual dependencies validated by Large Language Models (LLMs), and injects contextual co-occurrence pseudo-labels as external knowledge to guide detection. Without architectural changes, CODet consistently improves five state-of-the-art VLM-based detectors across two benchmarks, achieving notable gains (up to +2.3 AP on novel categories). Analyses further confirm its ability to encode meaningful contextual guidance, advancing open-world perception by bridging visual and textual co-occurrence knowledge.