5월(오월) -> May 다섯 달, 오개월-> five months (it means a period)

If you want to say with the word "다섯", it will be 다섯달. If you want to say with the word "오", it will be 오개월. Both 달 and 개월 refer to a month. 년 (year) is only used with 오 (일, 이, 삼, 사...), not 다섯.(하나, 둘, 셋, 넷..) I hope it helps :)