OMP is a fork of Pi[0], which is my preferred harness. Feels solid and minimal. I don't even use any extensions, skills, or modifications. Usually don't even use an AGENTS.md. Just create a small spec.md and/or plan.md for most experiments.
I'm chime in here as another Pi user, starting with OMP and then just ditching it for vanilla Pi + MCP/LSP from OMP as well as pi-cursor-sdk to get access to Composer 2.5 through Pi. I use this as my 'fast' research & report (through my project/codebase) or boilerplate/tightly-specced implementation agent that's available to my planning/orchestrator (GPT 5.5 if complex backend work, Fable/Opus if wanting the extended context or view/design heavy work).
Pi has been great in this role, as well as occasional use not as a sub-agent but by my own human hands when I want to interact with the Composer 2.5 model directly... which I do much less these days as I just use Codex 5.5 /fast. Pi's great though.
What's the benefit of using OMP over OpenCode?
Just the sheer amount of options in OMP overwhelmed me. But I also use both via ACP in Zed so the CLI itself doesn't matter much.