OASiS is an open-source Model Context Protocol (MCP) server that connects general-purpose LLM coding agents to eight independently developed finite element solvers (FEniCSx, deal.II, 4C Multiphysics, NGSolve, scikit-fem, Kratos Multiphysics, DUNE-fem, FEBio). It exposes a small set of shared tools for solver discovery, simulation preparation, mesh generation, run execution, multi-solver coupling, in-place solver development, and visualisation. A curated pitfall knowledge base, 26 cross-backend collation topics, and a mandatory critic-sub-agent workflow are used to verify simulations before they run.