Including OPC app into your theme using custom liquid blocks

 {%- when 'custom_liquid' -%}
     {{ block.settings.custom_liquid }}

  {
      "type": "custom_liquid",
      "name": "Custom Liquid",
      "settings": [
        {
          "type": "liquid",
          "id": "custom_liquid",
          "label": "Custom Liquid",
          "info": "Custom Liquid"
        }
      ]
    },