Oeselia mosaica is a fossil jawless fish and is the only species in its genus and in the family Oeseliidae (Oesel being the old name for Saaremaa Island in Estonia).

The family and genus were established along with the description of the species by Tiuu Märss in 2005.

The shapes of the scales are extremely variable, though are typically curved, and are all separated by fine grooves.

Trunk scales possess a quadrangular crown surface, where the posterior section tapers off or lowers to the anterior.

Scales appear flat and smooth under a microscope, but possess a very fine pattern.