Thermal donor formation was studied in oxygen enriched high resistive float zone silicon (FZ Si:Oi). Such substrates are used e.g. for radiation hard detectors or high voltage devices. RF Plasma hydrogenation (110 MHz, 50 W) was carried out at 250°C for 1 hour. Subsequent annealing was done at 450°C/air for up to 50 h. The plasma treated and annealed FZ Si:Oi samples were analyzed by spreading resistance probe, capacitance-voltage and DLTS measurements. It is shown that a rapid formation of donors can be observed in oxidized FZ Si:Oi, but in a somewhat different way than in Czochralski (Cz) Si. While in Cz Si the hydrogen enhanced formation of ‘old’ thermal double donors occurs under the applied processes, in FZ Si:Oi most probably the formation of new hydrogen related shallow donors can be assumed.