There is no difference in meaning between osadha and osadhī; both mean equally any medicine whether of herbs or other ingredients. Cp. e.g. AN.iv.100 (bījagāma-bhūtagāmā… osadhi-tiṇavanappatayo) Pv.ii.6#10 with Snp.296 (gāvo… yāsu jāyanti osadhā); DN.i.12, cp DN-a.i.98; Pv.iii.5#3; Pv-a.86; Ja.iv.31; Ja.vi.331 (? trsln. medicinal herb). Figuratively, ʻbalm of salvation’ (amatosadha) Mil.247. Osadhi-tārakā, star of healing. The only thing we know about this star is its white brilliance, SN.i.65; Iti.20 = AN.v.62; Vv.9#2; Pv.ii.1#10; cp. Pv-a.71 Vism.412. Childers calls it Venus, but gives no evidence other translators render it ʻmorning star’. According to Hindu mythology the lord of medicine is the moon (oṣadhīśa), not any particular star.
Vedic avaṣa + dhī: bearer of balm, comfort, refreshment