I'm using O365 A3. I am building a SharePoint Online site where I've created a workflow on a list on the top level site collection.
I've now created 5 sub-sites and I would like to re-use this workflow but don't know how. How can I achieve this?
Yes, It is suggested to create a reusable workflow or attach workflow with the content type from designer, if you want to use this workflow in other sites or subsites.
If you have created list workflow, here is good article that show how to reuse this list workflow in another site.
SharePoint 2013: Create Reusable Workflow on Content Type using SharePoint Designer 2013
Reusable workflows are not created for specific list or library but as the name suggests, they can be associated to any list or library. Here we will be creating reusable workflow on Content Type (CT) and this CT is used by multiple list or libraries. This allows our workflow to work for multiple list or libraries.
Select "Reusable Workflow" under "Workflows" and provide details- Name of the workflow Select CT from dropdown created in earlier steps Workflow Platform as "SharePoint 2013 Platform"
Then Start youe workflow codeing
Onces it done with the codeing, deploy that worklfow on SharePoint site.
Then it will create site feature with workflow name, activate that feature.
then attached workflow to list or library haveing same content type.
So it will doden't matter where youre list is on root site or subsite
Here's another article that may help package, deploy, and import list workflow into a different site