Complex sentence: Most modern oöids are aragonite (a polymorph of calcium carbonate); some are composed of high-magnesium calcite, and some are bimineralic (layers of calcite and aragonite).
Split into simpler sentences: Most modern oöids are aragonite (a polymorph of calcium carbonate). Some are composed of high-magnesium calcite, or are bimineralic (layers of calcite and layers of aragonite).