You can make certain modifications to the way a field is rendered with "Field Formatters", but they don't currently support string modification or text replacement. I suggest using a scripted field to get the value you want, if you write a script that replaces certain values with the string " OK" then Kibana will render that string instead and it will include the emoji.
Start by going to the index pattern you're interested in, click the "Scripted Fields" tab, and then "Add Scripted Field"
Then you can define your scripted field as a string that checks the value of one field and produced a different value with an emoji in it:
Now when you use this new field you will have emoji in your data
DISCLAIMER: Scripted fields are executed on every search, and if your data is massive or you have a relatively small cluster it might put unnecessary strain on Elasticsearch. If you find yourself reaching for scripted fields often it's generally recommended to do this type of transformation at ingestion time. Using something like Logstash or Elasticsearch ingest pipelines you can permanently add a field like this to your documents before they are stored in Elasticsearch.