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"OK" is an interjection which is often translated as "está bien", and "let" is a transitive verb which is often translated as "dejar". Learn more about the difference between "OK" and "let" below.
OK(
o
-
key
)An interjection is a short utterance that expresses emotion, hesitation, or protest (e.g., Wow!).
a. está bien 
OK, don't worry. I will take care of it.Está bien, no te preocupes. Yo me encargaré de eso.
b. de acuerdo 
OK, I will go to the party if you come with me.De acuerdo, iré a la fiesta si vienes conmigo.
c. okey  (colloquial) (Latin America)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Regionalism used in Latin America: all the countries in South America, Central America, and the Caribbean. Does not include Spain.
OK, let's go to the movies tonight!Okey, ¡vamos al cine esta noche!
d. vale  (colloquial) (Spain)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Regionalism used in Spain
OK! I will clean the kitchen. Just don't say it again.¡Vale! Limpiaré la cocina. Solo no insistas más.
e. órale  (colloquial) (Mexico)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Regionalism used in Mexico
OK, I'll see you tomorrow at 6.Órale, te veo mañana a las 6.
f. listo  (Latin America)
Regionalism used in Latin America: all the countries in South America, Central America, and the Caribbean. Does not include Spain.
Can you come help me this afternoon? - OK! What time?¿Puedes venir a ayudarme esta tarde? - Listo! ¿A qué hora?
An adjective is a word that describes a noun (e.g., the big dog).
3. (acceptable)
4. (all right)
An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
a. bastante bien 
Janet did OK in the exam. She thinks she'll pass.A Janet le fue bastante bien el examen. Cree que aprobará.
A transitive verb is a verb that requires a direct object (e.g., I bought a book.).
6. (to approve)
a. dar el visto bueno a 
Hopefully, the project will be OK'd at the meeting and we can start work on it next month.Esperemos que le den el visto bueno al proyecto en la reunión y que podamos empezar a trabajar en él el mes que viene.
let(
leht
)A transitive verb is a verb that requires a direct object (e.g., I bought a book.).
1. (to allow)
a. alquilar 
He is planning to let his apartment for three months.Está pensando alquilar su apartamento por tres meses.
A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).