1on·ly 
/ˈoʊnli/ adjective
always used before a noun /ˈoʊnli/ adjective : alone in a class or category : existing with no other or others of the same kind — usually used with the or a possessive (such as my, her, its, their, John's, etc.) ▪ He's the only [=one] man I've ever loved. ▪ They were the only people to survive the crash. ▪ You're not the only one who's worried about the future. [=other people are also worried about the future] ▪ You're the only person I trust. ▪ my only true friend ▪ He was her only brother. ▪ What a shame, your party is on the only day I can't come! ▪ Mondays are Kim's only days off from work. ▪ This is the only cleaning product you'll ever need! ▪ Keeping you safe is our only [=sole] concern. ▪ That was the only reason I didn't take the job. ▪ The only way out is through that window. ▪ It's the only possible answer. = It's the only answer possible. ▪ The only thing left for us to do is wait. [=there is nothing else we can do but wait] ▪ (informally) I can probably come Friday. The only thing [=the one problem] is that my car is in the shop. ▪ It's the first and only drug of its kind. — see also only child, the one and only at 3one
one of the only
: one of very few : one in a small class or category
▪ That was one of the only times I ever saw my father cry. ▪ This is one of the only places in the world where the plant is found.
▪ That was one of the only times I ever saw my father cry. ▪ This is one of the only places in the world where the plant is found.
the only
— used to emphasize that a particular person or thing should be the one chosen — usually + for ▪ This is the only book for serious collectors. ▪ She's the only person for the job.
