Hypselodoris orsinii has following characteristics :
- small size
- the dorsum is blue bordered in yellow with a median thin white line. The mantle is covered in small blue tubercles.
- each side of the foot is blue with a white or yellow line
- the rhinophores are uniform blue
- the gills are uniform blue too

In Mediterranean and Atlantic Ocean there are 6 similar blue dorids species. Here is a comparative table of characteristics and a comparative draw of the rhinophores and gills.

H. orsinii could be distinguished from Hypselodoris tricolor by :
- its dorsum : in H. orsinii it has only a thin white line, while in H. tricolor it has a solid median yellow line with some whitish yellow spots
- its gills : in H. orsinii they are uniform blue, while in H. tricolor they have a yellowish line on each gill

H. orsinii could be distinguished from Hypselodoris cantabrica by :
- its flanks : in H. orsinii they have only a white line, while in H. cantabrica they consist in numerous yellow spots
- its rhinophores : in H. orsinii they are uniform blue, while in H. cantabrica they have a yellow line
- its gills : in H. orsinii they are uniform blue, while in H. cantabrica they have 2 triangular yellow lines on each gill

H. orsinii could be distinguished from Hypselodoris villafranca by :
- its dorsum : in H. orsinii it has only a thin white line, while in H. villafranca it consists in a pattern of longitudinal yellow and/or white lines
- its rhinophores : in H. orsinii they are uniform blue, while in H. villafranca they have a white line
- its gills : in H. orsinii they are uniform blue, while in H. villafranca they have a white line on each gill

H. orsinii could be distinguished from Hypselodoris fontandraui by :
- its dorsum : in H. orsinii it has only a thin white line, while in H. fontandraui it has a median white line and some white spots
- its flanks : in H. orsinii they have only a whitish line, while in H. fontandraui they consist in a pattern of irregular white lines

H. orsinii could be distinguished from the northern variant of Hypselodoris bilineata by :
- its rhinophores : in H. orsinii they are uniform blue, while in H. bilineata they have a yellow line
- its gills : in H. orsinii they are uniform blue, while in H. bilineata they have 2 triangular yellow lines on each gill

Mediterranean