Can we index only few fields in an OpenSearch document?

Yes, you can use the "index" option controls to decide if fields are indexed. It accepts true or false with defaults being "true". Fields that are not indexed are typically not queryable. e.g:

PUT /<index-name>
{
  "mappings": {
    "properties": {
      "field1": {
        "type": "text",
        "index": false
      }
    }
  }
}