"If it is fine."
Translation:Os ydy hi'n braf.
os bydd... and the negative os na fydd... are the usual constructions - uncertainty is usually in the future rather than the present. If the present really is required, it is always os ydy..., never *os mae..., for some reason.
Where the Welsh expression uses os bydd..., the English tends to stick with the present 'if ... is/are ...'.