You can still be original with a non-original idea.
Ideas are a dime a dozen. The sooner people understand that, the better. Execution always matters -- if you can make that idea exciting, that's what draws people in. Whether or not they've seen it before becomes second when they like the story for what it is. I've read many stories with similar premises -- their executions still made me come back for more.
Oda isn't the first to do the "band of misfits come together to go on an adventure", and he won't be the last -- but his execution was enough to draw people's attention and have them love his work.