A new, very fast approximate method is presented to determine mass-loss rates of O-stars from Halpha_ line profiles. The method uses H and HeII departure coefficients from unified model atmospheres parametrized in a simple way as function of wind velocity together with photospheric NLTE line profiles as the inner boundary condition for a numerically exact radiative transfer solution to derive a wind contaminated Halpha_-profile. The method is also applied to Hgamma_ to determine stellar gravities corrected for wind emission. A detailed analytical discussion of Halpha_ line formation in O-star winds is given and it is demonstrated that former very simple approaches considering only optically thin wind emission lead to significant systematic errors. Scaling relations and generalized curves of growth are presented that connect mass-loss rate, terminal velocity, stellar parameters and Halpha_ equivalent width. The method is applied to samples of O-stars in the Galaxy, LMC and SMC and mass-loss rates are derived from Halpha_ in combination with terminal velocities measured from IUE and HST spectra. The results reveal that a tight empirical relation exists between the radius modified stellar wind momentum rate ˙(M)vinfinity_R_*_^0.5^ and the stellar luminosity. The variations of this relationship between the Galaxy, LMC and SMC are explained in terms of different abundances. Furthermore, for almost all objects with dense winds (mostly supergiants), the commonly used velocity field exponent β could be derived, indicating a typical value of β=~1. A comparison with the improved theory of radiation driven winds (as presented recently by Pauldrach et al. 1994) shows that the observed wind momentum-luminosity relationship can be understood qualitatively in terms of the theory. However, there exist significant systematic discrepancies as a function of effective temperature, luminosity class and wind performance number η=˙(M)vinfinity_c/L. We stress that these discrepancies would not have been detected with previous simplified Halpha_ -approaches. The deficiencies of the theory are discussed and suggestions for future improvements are made.