Ontologies are top-level entities that can be attached to projects that provide a template structure for labels.
For a detailed description about ontologies in the Encord platform go here.
A Project can only have a single Ontology attached to it, but one Ontology can be attached to multiple Projects.
Each ontology is identified using a unique <dataset_hash> - a unique ID that can be found within a Dataset in the Encord platform, as highlighted below.
To access the Ontology use the get_ontology() method, that allows you to work with its structure property. 
from encord import EncordUserClient

user_client: EncordUserClient = EncordUserClient.create_with_ssh_private_key(
    "<your_private_key>"
)

ontology = user_client.get_ontology("<ontology_hash>")
The structure comes as the OntologyStructure class.
Ontologies cannot be deleted using the SDK or the API. Use the Encord platform to delete an Ontology.

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View Ontology Structure

Use the following script to view the Ontology structure and all other Ontology attributes. 
from encord.user_client import EncordUserClient

# Define constants
SSH_PATH = "file-path-to-your-ssh-key"

user_client = EncordUserClient.create_with_ssh_private_key(
    ssh_private_key_path=SSH_PATH
)


ontology = user_client.get_ontology("unique-id-of-your-ontology")

print(ontology.ontology_hash)
print(ontology.title)
print(ontology.description)
print(ontology.created_at)
print(ontology.last_edited_at)
print(ontology.structure)