wp_dequeue_script not working in my plugin

wp_enqueue_script() also registers the script. So, you’ll also need to de-register it first. This code should work depending on how and where the script is enqueued.

function my_dequeue() {
    wp_deregister_script( 'ocmx-jquery' );
    wp_dequeue_script( 'ocmx-jquery' );
}
add_action( 'admin_enqueue_scripts', 'my_dequeue', 10 );

You might also need to play with the priority of the action.