“Oh, it was a no-brainer. I accepted right there on the spot.”
You recently were offered an opportunity to transfer to an office in another country. You're telling a friend about the offer. She asks if the decision was easy or difficult for you, so you tell her how easy it was.
Oh, it was a no-brainer. I accepted right there on the spot.
"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 "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.
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.
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.