Oh

"Oh" is a word that usually shows suprise. You use it when you find out some new information that you didn't expect:

Oh, you two know each other? I had no idea!

But in polite conversation, people sometimes use "Oh" in a subtle way. You can use it to appear surprised when someone does something nice for you. This makes you seem humble. For example, you can respond to a compliment using "Oh":

A: You're a good writer.

B: Oh, thank you!

Or if someone gives you a gift, you can seem surprised:

Oh, you got me something for my birthday? You shouldn't have! Thank you so much!

Good thinking.

When someone has a good idea, you can tell them "good thinking":

A: Hey, why don't we take a break.

B: Good thinking.

This phrase sounds friendly. You can use it with coworkers or friends.

that would be (nice/great/etc.)

Use the phrase "that would be ___" to accept someone's offer. For example:

A: Would you like something to drink?

B: Sure, that would be great.

A: I can watch your kids for you if you'd like to go out for a few hours.

B: Really? That would be wonderful!

You can use a variety of words that mean "good", including "wonderful", "amazing", "lovely", "fantastic", "neat", and others.