OmniCaptioner is a versatile visual captioning framework designed to generate detailed textual descriptions across various visual domains, including natural images, visual text, and structured visuals. It enhances visual reasoning with large language models (LLMs), improves image generation tasks, and allows for efficient supervised fine-tuning by converting pixel data into rich semantic representations. The framework aims to bridge the gap between visual and textual modalities through a unified multimodal pretraining approach.