Stress relaxation of drying colloids attracts our interest because the final mechanical properties of materials depend on colloidal structure. We use oil droplets as probes to visualize how local stress develops in the colloids near close packing. The colloids are condensed due to uniaxial evaporation, and the drying pressure compresses both colloids and oil droplets. The deformation of oil-colloids composite is observed under confocal fluorescence microscopy. During slow drying process, viscosity of the colloids is negligible, and the composite is under a hydrostatic state. For each moment, surface tension of oil-suspension interface and elasticity of the colloids are assumed to balance. The surface tension is measured separately and assumed as a constant during evaporation because we add saturated surfactant to the colloids. By knowing surface tension, strain of the composite, and packing fraction, the osmotic pressure of colloids is obtained. We find the pressure measured for poly (methyl methacrylate) colloids agrees with hard-sphere theory. We hope to apply the oil-colloids composite to study stress relaxation in soft colloids which cannot be predicted by theories.