おはよう！って英語でなんて言うの？

The easiest and most natural expression for a casual greeting is absolutely, "Good morning," no doubt about it. One slight variation that you see among families or people who live together would be, "How did you sleep?" or, "Did you sleep well?" This shows real interest in how the person is feeling.

Good morning. - Used in daily life for either personnel or business.
Wakey, Wakey, rise and shine! -This can be used for children/family/coworkers if you want to add some humor to start the day off and they are look/act tired. (recommended to only use this with close acquaintances.)

"Good morning" is definitely the most common English greeting in the morning.
It can be used for one individual person or for a group of people.

Some native speakers say "good morning to you" to make their greeting specifically for one person instead of a general greeting for a group of people.

You can simply say 'Morning' followed by politely asking the person how they slept.

Any of these get used in the morning

G'day means 'Good day' which is a contracted form used in Australia when greeting people any time of day.
Top of the morning is an Irish expression used as a friendly morning greeting.
"Top of the morning to you, Inspector."
'Morning' is the normal shortened version of 'Good morning' used in the UK.
"Morning Peter!"
"Hi, how are you doing?"

All of these may be used when greeting someone in the morning.
Good morning is commonly used as it is appropriate to use in formal and informal settings.

Good morning - This is most commonly used to greet people in the morning.