Github does not support searching the labels that way. The searching for issues is considered to be an "AND" instead of an "OR" .
Here's a repository used to try this out.
Example demonstrating github supports "AND" in search
My expectations
All the issues that are enhancement, feature or both should appear..
enhancement and feature
enhancement
feature
Actual result
Only the ones which were both were shown.
- enhancement and feature
Hence it does not support OR operator.
UPDATE
Another tedious hack around this could be remove the unwanted labels using a -(minus) symbol before the label. Look at the following example
label:enhancement -label:bug -label:foo -label:bar
This will list all the items not having foo, bar and bug as a label on them. This will work but can become tedious if labels are too many.
Filtering issues containing only the enhancements
UPDATE as of 2021
Now you can search using an OR operation. Please see the answer by @VonC
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…