Why is it OH- and not HO-?
OH- and HO- are both acceptable.  The charge is on the oxygen, of course.  One can argue cation then anion (salts) versus alphabetical (e.g., AuBe, which is brittle and weak rather than ductile, suggesting ionicity).

Organic formulas are C, H, then alphabetical.  Order may be altered to conform to structure (e.g., propane as $\ce{H3CCH2CH3}$) or convenience $\ce{H3C^{-}Li^{+}}$ (which is nowhere near the real structure of methyllithium, but is how it reacts).