High resolution underwater 3D scene reconstruction is crucial for various applications, including construction, infrastructure maintenance, monitoring, exploration, and scientific investigation. Prior work has leveraged the complementary sensing modalities of imaging sonars and optical cameras for opti-acoustic 3D scene reconstruction, demonstrating improved results over methods which rely solely on either sensor. However, while most existing approaches focus on offline reconstruction, real-time spatial awareness is essential for both autonomous and piloted underwater vehicle operations. This paper presents OASIS, an opti-acoustic fusion method that integrates data from optical images with voxel carving techniques to achieve real-time 3D reconstruction unstructured underwater workspaces. Our approach utilizes an "eye-in-hand" configuration, which leverages the dexterity of robotic manipulator arms to capture multiple workspace views across a short baseline. We validate OASIS through tank-based experiments and present qualitative and quantitative results that highlight its utility for underwater manipulation tasks.