The content of interstitially solved oxygen (Oi) in heavily boron doped silicon (9- 29 mcm) were measured by low temperature Fourier transform infrared (FTIR) spectroscopy. Therefor an alternative thinning technique for silicon is used: by alkaline potassium hydroxide etching (KOH) prepolished silicon specimens are thinned down to 8 - 60 microns. The optimal end thickness depends on the boron concentration which specifies the free carrier concentration. Specimens with three different boron concentrations (9/19/29 mcm) were examined. The results are compared with gas fusion analysis (GFA) measurements. Furthermore the precipitated oxygen Oi was measured for a RTA process (20s@1250°C) with subsequent growth steps (4h@780°C + 16h@1000°C).