Hi Linda,
This is a great question so I have made it a new topic.
The metal flanges on nibs are flexible so they can be adjusted to fit different sized nibs. Some pen makers create the flange with a specific nib in mind. This makes it easy for the user to insert the nib and not have to adjust anything. Many oblique holders are completely adjustable to whatever nib you like, but it does take some finagling to get the nib to fit securely.
For this reason, I like to buy my oblique holders with a specific nib in mind and just use it for that. There is a big difference between say a Brause EF66 which is tiny and a NikkoG.
The difference in the holders is really about how they fit in your hand and how they feel when you write. The majority of round holders feel similar but their weight and width can make a difference. I like the Curv-E holder which I designed with Edward Curran of Chic Obliques because it has a flat shape on top which allows me to rest my finger on a flat surface and gives me more feedback. But it is all personal.
If you search through the forum, there are lots of posts about oblique holders. And a large list of oblique makers. John Neal and Pen & Ink Arts also carry a wide range, many of which can accommodate a range of nibs with a little tweaking.
You may like some of these articles:
Criteria for Selecting an Oblique How to Adjust an ObliqueThis one has a great video by
@YokePenCo Hope that helps!