OASIS: On-Demand Hierarchical Event Memory for Streaming Video Reasoning

Streaming video reasoning requires models to operate in a setting where history grows without bound while meaningful evidence remains scarce. In such a landscape, relevant signal is like an oasis -- small, critical, and easily lost in a desert of redundancy. Enlarging memory only widens the desert; aggressive compression dries up the oasis. The real difficulty lies in discovering where to look, not how much to remember. We therefore introduce OASIS, a novel framework for streaming video reasoning that tackles this challenge through structured, on-demand retrieval. It organizes streaming history into hierarchical events and performs reasoning as controlled refinement -- short-context inference first, followed by semantically grounded retrieval only when uncertainty arises. As the retrieval is driven by high-level intent rather than embedding similarity, the retrieve memory is substantially more accurate and less noisy. Additionally, the mechanism is plug-and-play, training-free, and compatible with any streaming MLLM. Experiments across multiple benchmarks show that OASIS achieves strong gains in long-horizon accuracy and compositional reasoning with far less memory budget.