Your ontology already exists.
Ontology always exists, but the one you have today may be messy or nonsensical. If you were trying to understand the ontology of your grocery store, your map might look like this at first:
By asking your customers for words they think about within a grocery store, your map could grow to reflect overlapping and related terms.
If you were choosing words for the aisle and department signs or the website, this exercise would help you along.