This is a fast converter from POLY files (Osmosis Polygon Filter File Format) to GeoJSON.
Go to https://github.com/frafra/poly2geojson/releases to download a compiled Linux binary for x86_64 or use cargo to build and install it:
cargo install poly2geojson
Cargo should be installed.
cargo build --release
poly2geojson < test.poly > test.geojson