This is the 3rd most frequent Finnish word.
oli
"Was" (past tense of "to be").
Here, 'oli' functions as the past tense of the verb 'olla' (to be) describing a state of being.
Hän oli onnellinen.
He/She was happy.
In this case, 'oli' is used impersonally to describe a general past observation or state related to the event mentioned.
Oli mukavaa nähdä sinut.
It was nice to see you.
Here, 'oli' again represents the past tense of 'olla' (to be) emphasizing the description of the task's nature.