A biosensor for putrescine containing a sensing layer with an optical oxygen probe based on ruthenium complex and the enzyme diamine oxidase from pea is described. The diamine oxidase was pre-immobilised on broken micro-beads modified with a ferrofluid. The pre-immobilised enzyme and ruthenium complex were both incorporated into the UV-cured inorganic-organic hybrid polymer ORMOCER and deposited on a lens to form a sensitive layer of 210 µm in thickness. The sensitivity to the putrescine concentration determined under air saturation was between 3.50 µs L mmol −1 and 4.50 µs L mmol −1 in a hundred experiments conducted intermittently over a one year period. With the oxygen concentration increasing from 10 % to 100 % of DO (dissolved oxygen), the biosensor sensitivity decreased from 6.87 µs L mmol −1 to 0.70 µs L mmol −1 and its dynamic range increased from 0.10 mmol L −1 to 1.75 mmol L −1 . To estimate the behaviour of the putrescine sensor in parametric space, a mathematical model of the reaction-transport processes inside the sensing layer was developed. The model revealed the qualitative relations between the sensor analytical features, the characteristics of the sensitive layer and concentrations of substrates. The results of the mathematical modelling may serve as guidelines in the design of optodes for specific applications.