OCP does not refer to open/closed principle, it's a reference to OCP, or an Outside Context Problem, from sci fi author Iain M Banks' writing. An OCP is a force that defies all attempts to understand it and represents an existential threat to humanity - "a civilization encounters an OCP in much the same way as a sentence runs into a full stop." Bungie are HUGE Banks fans; they've said in their tribute to him when he died of cancer that they based the Halo Rings on the Orbitals from his Culture series.