The present paper reports on deep research investigations towards the direct design of hollow section shape beam-columns by means of the Overall Interaction Concept (O.I.C.). It details how this approach can simply yet effectively handle the influence of both local and global instabilities on the plastic capacity of the member. The results of 12 beam-column tests on members up to 5.0 m are presented, as well as many other ones collected from the literature. Application of the O.I.C. to characterize these experimental results allows to evidence that the concept is capable of directly accounting for the most relevant parameters, as well as of clearly identifying the remaining ones to be accounted for in the reduction factor-slenderness interaction curves. Besides, validated shell F.E. models have been used extensively to gather a pool of numerical reference results, in the perspective of deriving interaction curves for design practice. Results demonstrate that interaction curves shall be made dependent i) of the bending moment distribution, ii) of the cross-section slenderness, iii) of the relative level of axial compression, and iv) of the tube manufacturing process.