"o" meaning in Volapük

{
  "head_templates": [
    {
      "args": {
        "1": "vo",
        "2": "particles"
      },
      "expansion": "o",
      "name": "head"
    }
  ],
  "lang": "Volapük",
  "lang_code": "vo",
  "pos": "particle",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with raw sortkeys",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Volapük entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Volapük particles",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "english": "Dear friends",
          "text": "O flens löfik!",
          "type": "example"
        }
      ],
      "glosses": [
        "vocative case particle"
      ],
      "id": "en-o-vo-particle-5fXa7afZ",
      "links": [
        [
          "vocative case",
          "Unsupported titles/vocative`lowbar`case#English"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/o/"
    }
  ],
  "word": "o"
}

{
  "head_templates": [
    {
      "args": {
        "1": "vo",
        "2": "particles"
      },
      "expansion": "o",
      "name": "head"
    }
  ],
  "lang": "Volapük",
  "lang_code": "vo",
  "pos": "particle",
  "senses": [
    {
      "categories": [
        "Pages with raw sortkeys",
        "Volapük entries with incorrect language header",
        "Volapük lemmas",
        "Volapük particles",
        "Volapük terms with IPA pronunciation",
        "Volapük terms with usage examples"
      ],
      "examples": [
        {
          "english": "Dear friends",
          "text": "O flens löfik!",
          "type": "example"
        }
      ],
      "glosses": [
        "vocative case particle"
      ],
      "links": [
        [
          "vocative case",
          "Unsupported titles/vocative`lowbar`case#English"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/o/"
    }
  ],
  "word": "o"
}