In the context of mixed systems that seek to smoothly merge physical and digital worlds, designing and prototyping interaction involves physical and digital aspects of mixed objects. However, even though mixed objects are recurrent in the literature, none of the existing prototyping tools explicitly supports this object level. Moreover, designers have to use distinct tools, on the one hand, tools for designing ideas and on the other hand tools for prototyping them: this makes the design process difficult. To help alleviate these two problems, we present OP (Object Prototyping), a toolkit that provides a new programming model focusing on mixed objects and allows us to seamlessly go back and forth from conceptual ideas to functional physical prototypes, making the iterative design process smooth and integrated. Indeed, OP is explicitly based on an existing conceptual design model, namely the Mixed Interaction Model that has been shown to be useful for exploring the design space of mixed objects. Our user studies show that, despite its threshold, designers and developers using OP can rapidly prototype functional physical objects as part of a design process deeply intertwining conceptual design with prototyping activities.