It is one of those masculine nouns which has a feminine plural ending: עונות ah-vo-NOTE.
This is also an example of a vav with a cholem that does NOT act as a long vowel. When you read, you will know because there must be a consonant-vowel pattern throughout the word. A cholem vav would place two vowels next to each other. (qamets under the ayin followed by cholem vav in the singular.)
spelling: C: ayin V: qamets C: vav V: cholem C: nun sofeet (singular) עון
spelling C: ayin V: cheteph patach C: vav V: cholem C: nun V: cholem vav C: tav (plural) עונות
In the latter, we have a vav with a simple cholem, as cholem appears with any other letter, and then we have the vowel cholem vav. Makes an interesting word all around.
I may be the only nut out here that is fascinated and studies these things. But for what it's worth: there ya' go!