In this work, we present a new optical routing framework, O-Router for future low-power on-chip optical interconnect integration utilizing silicon compatible nano-photonic devices. We formulate the optical layer routing problem as the minimization of total on-chip optical modulator cost (laser power consumption) with Integer Linear Programming technique under various detection constraints. Key techniques for variable number reduction and routing speedup are also explored and utilized. O-Router is tested on optical netlist benchmarks modified from top global nets of ISPD98/08 routing benchmarks. O-Router experimental results are compared with conventional minimum spanning tree algorithm, demonstrating an average of over 50% improvement in terms of total on-chip optical layer power reduction.