In this paper, we propose an adaptive software-defined optical sensor network (SDOSN) architecture that is capable to host programmable sensors ranging from several thousands to several millions. Sensing process is of low cost, but highly accurate and capable of closely approaching the optical channel capacity. The proposed SDOSN architecture is interoperable with existing optical networks infrastructure. Both the existing dark fibers can be used together with the fiber infrastructure being in use. Additionally, the proposed SDOSN is programmable at runtime to change sensor network topology, active sensors, and their functions to accommodate current sensor network objectives and applications. The proposed sensor network is cost-effective and leverages the emerging virtual network technologies and software defined network paradigms. Additionally the orbital angular momentum (OAM) is used as an additional degree of freedom (DOF) with the purpose to: (i) secure sensor data and (ii) provide a new DOF, OAM multiplexing, to support a larger number of sensor nodes.