When I buy a ticket, will it pay out at the odds I bought it at, or the odds when the pool closed?

Your payout will be whatever the odds are when the race starts. NOT what the odds are when you make your bet. Horse  racing is different than sports betting where the odds are locked in when you make your bet. That's why it's important to not only look at where the odds are at the time, but to get an idea of where they are likely to end up at post time.

In the US, the ticket will pay out at the odds when the race began. 

I believe that in the UK, they pay out at the odds you bought it at.