If it's naturally occurring in a food, and not in pill form, whats the difference between oat fiber and say, celery fiber?

I believe that in general(, the more fibre you get whilst eating the better. So sure if you are getting most of your fibre during other meals and are about to eat a sandwich. It is better to also have fibre during that eating session.

As with anything it is just general, but less processed / higher fibre / whole grain food is better and (subjectively) much tastier. After only eating higher whole grain food, going back to the more processed version, you realise just how lacking it is (opinion)

Get your fiber from a well-balanced diet.