1. Introduction

2. Data description

3. Analysis method

4. Fundamental parameters
4.1. Effective temperatures
4.2. The T_eff scale of O2 stars
4.3. Gravities
4.4. Helium abundances
4.5. Micro turbulence
4.6. Wind parameters

5. The mass discrepancy
5.1. Chemically homogeneously evolving O2 stars
5.2. Large sample trends

6. The evolutionary status of N11
6.1. LH9 and LH10 in N11
6.2. The ages of LH9 and LH10
6.3. Sequential star formation?

7. Summary and conclusions