If you want custom order number, instead default generate, this guide can help you . Or use 100% offline order number

add_filter('op_get_login_cashdrawer_data',function($session_response_data){
    /*
     For example: {{register_id}}-{{store_id}}-{{order_index}}
     explain:
    '{{year}}' : Current year in 4 digits,
    '{{month}}' : current month in 2 digits,
    '{{date}}' : current date in 2 digits,
    '{{hour}}' : current hour in 2 digits,
    '{{minute}}' : current minute in 2 digits,
    '{{second}}' : current second in 2 digits,
    '{{register_id}}' : ID if logged in cashdrawer,
    '{{store_id}}' : ID of current store/outlet,
    '{{order_index}}': Sequential order number of the day starting from 1 (resets every login).
    */
    $session_response_data['setting']['pos_sequential_number_format'] = '{{register_id}}-{{store_id}}-{{order_index}}'; 
    //$session_response_data['setting']['pos_use_offline_order_number'] = 'yes'; // uncomment if  you want use full is offline order number  
    return $session_response_data;
 }, 99, 1);