"-o-" meaning in Polish

{
  "etymology_templates": [
    {
      "args": {
        "1": "pl",
        "2": "inh",
        "3": "zlw-opl>-o->interfix",
        "id": "interfix",
        "tree": "1"
      },
      "expansion": "Etymology tree\nProto-Slavic *-o-\nOld Polish -o-\nPolish -o-",
      "name": "etymon"
    },
    {
      "args": {
        "1": "Inherited"
      },
      "expansion": "Inherited",
      "name": "glossary"
    },
    {
      "args": {
        "1": "pl",
        "2": "zlw-opl",
        "3": "-o-",
        "4": "",
        "5": "",
        "g": "",
        "g2": "",
        "g3": "",
        "id": "",
        "lit": "",
        "nocat": "",
        "pos": "",
        "sc": "",
        "sort": "",
        "tr": "",
        "ts": ""
      },
      "expansion": "Old Polish -o-",
      "name": "inh"
    },
    {
      "args": {
        "1": "pl",
        "2": "zlw-opl",
        "3": "-o-"
      },
      "expansion": "Inherited from Old Polish -o-",
      "name": "inh+"
    }
  ],
  "etymology_text": "Etymology tree\nProto-Slavic *-o-\nOld Polish -o-\nPolish -o-\nInherited from Old Polish -o-.",
  "head_templates": [
    {
      "args": {
        "1": "pl",
        "2": "interfix"
      },
      "expansion": "-o-",
      "name": "head"
    }
  ],
  "hyphenation": [
    "o"
  ],
  "lang": "Polish",
  "lang_code": "pl",
  "pos": "interfix",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with 17 entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Polish entries with etymology trees",
          "parents": [
            "Entries with etymology trees",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Polish entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Polish interfixes",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "deszcz + -o- + mierzyć → deszczomierz",
          "type": "example"
        }
      ],
      "glosses": [
        "used in compound words"
      ],
      "id": "en--o--pl-interfix-yYkJPb0G",
      "related": [
        {
          "word": "-i-"
        },
        {
          "word": "-u-"
        },
        {
          "word": "-y-"
        }
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/ɔ/"
    },
    {
      "rhymes": "-ɔ"
    },
    {
      "homophone": "o"
    },
    {
      "homophone": "o-"
    },
    {
      "homophone": "-o"
    }
  ],
  "word": "-o-"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "pl",
        "2": "inh",
        "3": "zlw-opl>-o->interfix",
        "id": "interfix",
        "tree": "1"
      },
      "expansion": "Etymology tree\nProto-Slavic *-o-\nOld Polish -o-\nPolish -o-",
      "name": "etymon"
    },
    {
      "args": {
        "1": "Inherited"
      },
      "expansion": "Inherited",
      "name": "glossary"
    },
    {
      "args": {
        "1": "pl",
        "2": "zlw-opl",
        "3": "-o-",
        "4": "",
        "5": "",
        "g": "",
        "g2": "",
        "g3": "",
        "id": "",
        "lit": "",
        "nocat": "",
        "pos": "",
        "sc": "",
        "sort": "",
        "tr": "",
        "ts": ""
      },
      "expansion": "Old Polish -o-",
      "name": "inh"
    },
    {
      "args": {
        "1": "pl",
        "2": "zlw-opl",
        "3": "-o-"
      },
      "expansion": "Inherited from Old Polish -o-",
      "name": "inh+"
    }
  ],
  "etymology_text": "Etymology tree\nProto-Slavic *-o-\nOld Polish -o-\nPolish -o-\nInherited from Old Polish -o-.",
  "head_templates": [
    {
      "args": {
        "1": "pl",
        "2": "interfix"
      },
      "expansion": "-o-",
      "name": "head"
    }
  ],
  "hyphenation": [
    "o"
  ],
  "lang": "Polish",
  "lang_code": "pl",
  "pos": "interfix",
  "related": [
    {
      "word": "-i-"
    },
    {
      "word": "-u-"
    },
    {
      "word": "-y-"
    }
  ],
  "senses": [
    {
      "categories": [
        "Pages with 17 entries",
        "Pages with entries",
        "Polish 1-syllable words",
        "Polish entries with etymology trees",
        "Polish entries with incorrect language header",
        "Polish interfixes",
        "Polish lemmas",
        "Polish terms derived from Old Polish",
        "Polish terms derived from Proto-Slavic",
        "Polish terms inherited from Old Polish",
        "Polish terms inherited from Proto-Slavic",
        "Polish terms with IPA pronunciation",
        "Polish terms with audio links",
        "Polish terms with homophones",
        "Rhymes:Polish/ɔ",
        "Rhymes:Polish/ɔ/1 syllable"
      ],
      "examples": [
        {
          "text": "deszcz + -o- + mierzyć → deszczomierz",
          "type": "example"
        }
      ],
      "glosses": [
        "used in compound words"
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/ɔ/"
    },
    {
      "rhymes": "-ɔ"
    },
    {
      "homophone": "o"
    },
    {
      "homophone": "o-"
    },
    {
      "homophone": "-o"
    }
  ],
  "word": "-o-"
}