This is the 6110th most frequent Japanese word.
オシドリ
Mandarin duck.
The word 'オシドリ' is used here to directly refer to the species of bird known as the mandarin duck.
オシドリは美しい鳥です。
The mandarin duck is a beautiful bird.
In this context, 'オシドリ' metaphorically describes a harmonious and loving relationship between couples, referencing the species' reputed pair bonding habits.
彼らはオシドリ夫婦のように仲が良いです。
They get along well, like a pair of mandarin ducks.
Here, 'オシドリ' is used literally to describe the observer seeing this specific bird species in a location.