(someone) accepts an offer

"Accepting" something means saying "Yes" or "OK" in respnse. You can:

accept an invitation

accept a challenge

accept an idea

accept an offer

An "offer" is something that a person tries to give you. Here are some common "offers" that people make:

offer to help

offer (someone) a job

offer a salary

So "accepting an offer" means taking something that someone tries to give you, like a job. 

(a decision) is a no-brainer

A "no-brainer" is a decision that's very easy to make. You are able to decide immediately without thinking about it.

For example, if someone offers you a job at twice your current salary, accepting the job might be "a no-brainer". You can talk about it like this:

It was a no-brainer. I said yes immediately.

You can remember "no-brainer" by imagining a decision that's so easy to make, even a person without a brain can make the right choice.

(do something) on the spot

To do something "on the spot" means to do it immediately. You can use "on the spot" to talk about making a decision quickly. For example:

I asked her to marry me, and she said yes on the spot.

It's common to add "right there":

After I heard that, I decided "I've had it!" and I quit right there on the spot.

Oh, (answer)

People add "oh" to the beginning of a sentence for lots of different reasons. One way that English speakers use "Oh" is when they have a strong reaction to someone's question. For example:

A: Are you two a couple?

B: Oh, no. No way! We're just friends.

A: Is that a nice neighborhood?

B: Oh, yes. It's one of the best neighborhoods in the city.

So you can use "Oh" when you're really sure about your answer to a question. But a confusing point is that you can also say "Oh" when you're not sure how to answer a question. In that case, you might pause after "Oh":

A: Do you want to invite the Petersons over later this week?

B: Oh... well, maybe. But I'm pretty busy this week.