"-o" meaning in Romanian

{
  "etymology_templates": [
    {
      "args": {
        "1": "ro",
        "2": "sla-pro",
        "3": "-"
      },
      "expansion": "Proto-Slavic",
      "name": "der"
    }
  ],
  "etymology_text": "From Proto-Slavic.",
  "head_templates": [
    {
      "args": {
        "1": "ro",
        "2": "suffix"
      },
      "expansion": "-o",
      "name": "head"
    }
  ],
  "lang": "Romanian",
  "lang_code": "ro",
  "pos": "suffix",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Romanian entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "mamă (“mother”) + -o → mamo",
          "type": "example"
        },
        {
          "text": "focă (“seal”) + -o → foco",
          "type": "example"
        }
      ],
      "glosses": [
        "Vocative singular (feminine)"
      ],
      "id": "en--o-ro-suffix-PrZH1~3N",
      "related": [
        {
          "raw_tags": [
            "used for masculine and neuter adjectives or nouns"
          ],
          "tags": [
            "vocative"
          ],
          "word": "-e"
        }
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "word": "-o"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "ro",
        "2": "sla-pro",
        "3": "-"
      },
      "expansion": "Proto-Slavic",
      "name": "der"
    }
  ],
  "etymology_text": "From Proto-Slavic.",
  "head_templates": [
    {
      "args": {
        "1": "ro",
        "2": "suffix"
      },
      "expansion": "-o",
      "name": "head"
    }
  ],
  "lang": "Romanian",
  "lang_code": "ro",
  "pos": "suffix",
  "related": [
    {
      "raw_tags": [
        "used for masculine and neuter adjectives or nouns"
      ],
      "tags": [
        "vocative"
      ],
      "word": "-e"
    }
  ],
  "senses": [
    {
      "categories": [
        "Romanian entries with incorrect language header",
        "Romanian lemmas",
        "Romanian suffixes",
        "Romanian terms derived from Proto-Slavic"
      ],
      "examples": [
        {
          "text": "mamă (“mother”) + -o → mamo",
          "type": "example"
        },
        {
          "text": "focă (“seal”) + -o → foco",
          "type": "example"
        }
      ],
      "glosses": [
        "Vocative singular (feminine)"
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "word": "-o"
}