I'm working on a project on osclass 8.2.1 with sigma theme. From what I see, osclass custom fields are called on item_post by ItemForm::plugin_post_item();
Unfortunately, this management does not only show the custom fields on the page, but also the hook ofplugins and therefore, by removing it, I would also remove the plugins that work on the "item_post" page. In my project I only have one custom field (custom field called age) and my need is to insert it into item_post "manually" excluding it from the same one that is displayed by the hook. What do you advise me to do?

You are not able to change how custom fields are shown, but it's possible to customize position of plugin data by using variants.

https://docs.osclasspoint.com/item-publish-edit-hooks-split

(Advanced topic)

Hello and thanks for your answer MB Themes 

I'm aware that I can change the position, but that's not really what I want. I think another solution could be:

remove ItemForm::plugin_post_item() and manually add the plugin code I want. Of course I can then add the custom field wherever I want on the page. What do you think of this as a solution? maybe using Hooks Manager Plugin?

That is not possible in all scenarios as plugin data are dependand on category selection.