Out-of-distribution (OOD) detection aims to distinguish whether detected objects belong to known categories or not. Existing methods extract OOD samples from In-distribution (ID) data to regularize the model’s decision boundaries. However, the decision boundaries are not adequately regularized due to the model's lack of knowledge about the distribution of OOD data. To address the above issue, we propose an Adaptive Prompt Learning framework via Gaussian Outlier Synthesis (APLGOS) for OOD detection. Specifically, we leverage the Vision-Language Model (VLM) to initialize learnable ID prompts by sampling standardized results from pre-defined Q\&A pairs. Region-level prompts are synthesised in low-likelihood regions of class-conditional gaussian distributions. These prompts are then utilized to initialize learnable OOD prompts and optimized with adaptive prompt learning. Also, OOD pseudo-samples are synthesised via gaussian outlier synthesis. Similarity score between prompts and images is utilized to calculate contrastive learning loss in high-dimensional hidden space. The aforementioned methodology regularizes the model to learn more compact decision boundaries for ID and OOD categories. Extensive experiments show that our proposed method achieves state-of-the-art performance with less ID data on four mainstream datasets.