“You do not have sufficient permissions to access this page” upon accessing my newly created plugin page

add_options_page("OSCommerce Product Display", "OSCommerce Product Display", 1, "OSCommerce Product Display", "oscimp_admin");

should be

add_options_page( 'OSCommerce Product Display', 'OSCommerce Product Display', 'administrator', 'os_commerce_display_main', 'oscimp_admin' );

Solution is untested. Docs: Roles and Capabilities, add_options_page

The 3rd parameter is capability, not user level or whatever you were thinking. I set it to admin, see the docs on roles and capabilities if you want that setup differently. The other error you made was having spaces in your menu slug, it might have worked that way, but it can create a mess. You should also take a very thorough look at the WordPress Coding Standards if you want to do plugin development.

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