1 MOTIVATION AND GOALS  
1.1 Is there a magnetic desert for O-type stars?  
1.2 Can we detect magnetic fields in old main sequence O-type stars, and thereby constrain models of the evolution of their surface magnetism?  
1.3 Paper structure  
2 OBSERVATIONS  
3 BAYESIAN MODELLING  
3.1 Spectroscopic binaries  
3.2 Odds ratios  
3.3 Probability density function  
4 STATISTICAL RESULTS  
4.1 Population analysis  
4.2 Completeness of the MiMeS O-star survey  
5 DISCUSSION  
5.1 Observational bias in the detection of old magnetic stars  
5.2 Magnetic desert  
6 CONCLUSIONS