Now it can be told. Oscar Hammerstein II was heterosexual. And a helluva nice guy. Yes, after the recent deluge of biograffiti, it's a pleasure to find at least one giant of the American musical theater to be just what lie seemed to be in his scripts and lyrics (to music by Romberg, Kern, and Rodgers)--an optimist, a romantic, a soft-edged square. Fordin probably needn't have been quite so blandly benign in his anecdotal account of ""Ockie's"" peak-valley-peak career as wunderkind of the Twenties (Showboat), Hollywood/Broadway has-been of the Thirties (""He can't write his hat,"" they said), and renewed creative-commercial force as the kindly half of Rodgers & Hammerstein. The extent of Hammerstein's role as groundbreaker (was Kern the brains?) remains fuzzy. Few critical judgments are attempted--the ""lark who is learning to pray"" excesses of sentimentality are sashayed around--while most of the analyses of specific songs are lifted from Hammerstein's own foreword to his Lyrics. And Rodgers seems to have been unavailable for comment. Still, this is a pleasantly plain and moderately intelligent telling, most effective in its offstage moments (family members speak freely)--privileged Manhattan childhood as scion of a theatrical dynasty, slow disentanglement from a bad first marriage to embrace true-love Dorothy (also unhappily wed), a genially selfish triumph as husband and father. ""The theater ain't for you, kid. You got too much class!"" So Mae West told stage manager Ockie in 1918. Well, Fordin's earnest tribute certainly doesn't have too much class, but it has the opening night of Oklahoma, Paul Robeson singing ""Ol' Man River"" in a hushed rehearsal hall--enough bright, golden hazes to provide some enchanted evenings to casual rememberers (if not students) of musicals that sang.