Most Osclass users are looking for custom fields with multiple selections on checkbox which are searchable …
There is a lot of Forums about that Osclass missing feature. One of this Forum offer to update the Field.form.class.php file of Osclass core (https://forums.osclass.org/development/how-to-have-multiple-checkboxes-for-one-custom-field/) … Not the best solution for upgrading Osclass version later on …

One more suggestion : if the user doesn't check one checkbox option, the option will appear on the listing page, showing with a X. A great feature will be to remove the unavailable options on the listing page.