Shogakukan Unabridged Dictionary of the Japanese Language (2nd Edition)

Notes

Kana forms that reflect the kō/otsu vowel distinction of Old Japanese are indicated in the Jōdai Tokushu Kanazukai （上代特殊仮名遣い；Special Kana Orthography for Old Japanese) section, with kō-category forms printed in blue and otsu-category forms in red.
おとめ 【少女・乙女】　　【上代特殊仮名遣い】　ヲト　メ
In general this information is provided only for single words, not for compounds or for proper nouns.
In cases where examples of both kō and otsu orthography have been attested, both forms are given. In cases where there are difficulties in classifying a form as kō or otsu, a brief annotation is provided to that effect.