Size: 3-4mm
Basic colour: Dark metallic blue and orange-yellow
Pattern colour: Elytra with orange-yellow lateral margins.
Number of spots: None
Pronotoum: Dark metallic blue
Leg colour: Dark metallic blue
Confusion species: Hydrothassa hannoveriana
, Confusion species: Hydrothassa marginella

Size: 3.5-5.0mm
Basic colour: Metallic blue or greenish.
Pattern colour: None
Number of spots: None
Other colour forms: Sometimes
Pronotoum: As basic colour
Leg colour: As basic colour

Size: 4.0-4.5mm
Basic colour: Metallic blue.
Pattern colour: None
Number of spots: None
Pronotoum: As basic colour
Leg colour: As basic colour
Confusion species: Oulema erichsoni
, Confusion species: Oulema obscura

The taxonomic status of O. septentrionis in Ireland is currently under review as there is evidence from dissections of the aedeagal flagellum that specimens from Ireland and Normandy are a melanic form of O. melanopus. The final outcome of this work is awaiting publication.

Size: 3.0-4.2mm
Basic colour: Metallic blue, sometimes greenish or almost black.
Pattern colour: None
Number of spots: None
Other colour forms: Sometimes
Pronotoum: As basic colour
Leg colour: As basic colour
Confusion species: Oulema erichsoni
, Confusion species: Oulema septentrionis

Size: 4.0-4.5mm
Basic colour: Dark blue
Pattern colour: None
Number of spots: None
Pronotoum: As basic colour
Leg colour: As basic colour
Confusion species: Oulema obscura
, Confusion species: Oulema septentrionis