For a graph \( G = (V, E) \), a non-empty set \( S \subseteq V \) is a global offensive alliance (respectively, global strong offensive alliance) if for every vertex \( v \in V – S \), at least half of the vertices in its closed neighborhood are in \( S \) (respectively, a strict majority of its closed neighborhood are in \( S \)). The global offensive alliance number \( \gamma_o(G) \) (respectively, global strong offensive alliance number \( \gamma_{\hat{o}}(G) \)) is the minimum cardinality of a global offensive alliance (respectively, global strong offensive alliance) of \( G \). In this paper, we determine an upper bound on each parameter for bipartite graphs without isolated vertices. More precisely, we show that \( \gamma_o(G) \leq \frac{n – \ell + s}{2} \) and \( \gamma_{\hat{o}}(G) \leq \frac{n + \ell}{2} \), where \( n \), \( \ell \), and \( s \) are the order, the number of leaves, and the support vertices of \( G \), respectively. Moreover, extremal trees attaining each bound are characterized.