oestrus vs estrus

oestrus and estrus both are nouns.

Word	Noun	Adjective	Verb	Adverb
oestrus	Yes	No	No	No
estrus	Yes	No	No	No

As nouns, oestrus and estrus are synonyms defined as:

oestrus and estrus: applies to nonhuman mammals: a state or period of heightened sexual arousal and activity

Other synonyms of oestrus include heat, rut.

oestrus (noun)	estrus (noun)
applies to nonhuman mammals: a state or period of heightened sexual arousal and activity	applies to nonhuman mammals: a state or period of heightened sexual arousal and activity