MMOU: A Massive Multi-Task Omni Understanding and Reasoning Benchmark for Long and Complex Real-World Videos

TL;DR

This paper introduces MMOU, a comprehensive benchmark for evaluating multimodal understanding and reasoning in long, complex videos across visual, audio, and textual modalities, revealing significant gaps in current models' capabilities.

Contribution

The paper presents MMOU, a large-scale, high-quality benchmark with diverse questions and videos, designed to systematically assess and analyze multimodal reasoning in real-world, long-form videos.

Findings

Current models perform poorly on long videos, with accuracy below 65%.

Models struggle to integrate evidence across modalities and over time.

Analysis reveals systematic failure modes in multimodal reasoning.

Abstract

Multimodal Large Language Models (MLLMs) have shown strong performance in visual and audio understanding when evaluated in isolation. However, their ability to jointly reason over omni-modal (visual, audio, and textual) signals in long and complex videos remains largely unexplored. We introduce MMOU, a new benchmark designed to systematically evaluate multimodal understanding and reasoning under these challenging, real-world conditions. MMOU consists of 15,000 carefully curated questions paired with 9038 web-collected videos of varying length, spanning diverse domains and exhibiting rich, tightly coupled audio-visual content. The benchmark covers 13 fundamental skill categories, all of which require integrating evidence across modalities and time. All questions are manually annotated across multiple turns by professional annotators, ensuring high quality and reasoning fidelity. We evaluate 20+ state-of-the-art open-source and proprietary multimodal models on MMOU. The results expose substantial performance gaps: the best closed-source model achieves only 64.2% accuracy, while the strongest open-source model reaches just 46.8%. Our results highlight the challenges of long-form omni-modal understanding, revealing that current models frequently fail to apply even fundamental skills in long videos. Through detailed analysis, we further identify systematic failure modes and provide insights into where and why current models break.