Octonion algebras are non-associative algebras endowed with a non-degenerate multiplicative quadratic form, which over fields determines the algebra completely, but not over rings in general. Connecting to the talk given by P. Gille, and reporting on a joint work with him, I will show how to construct all octonion algebras having the same quadratic form. The key role is played by the triality phenomenon, as it relates certain $\mathbf G_2$-torsors to a classical construction of alternative algebras.