You can access a specific key with the array notation:
op_hours[key].push('shark')
where key === "thu"
for example
would give you:
op_hours = {
"mon": ["apple","orange"],
"tue": ["table", "chair"],
"wed": ["shark", "dolphin", "jellyfish"],
"thu": ["shark"],
"fri": [],
"sat": [],
"sun": []
}