Each tenant is a distinct, unique, site to monitor in Oh Dear.
From the outside, a "wildcard domain" doesn't typically exist, so if they are on subdomains, you would have to monitor each subdomain independently in Oh Dear. It'll have some overlap when it comes to SSL certificates, but at the same time, each domain could have a certificate failure if it points to a new DNS or a new webserver.
Ideally, you monitor each tenant (subdomain) independently. It can have its own certificate that expires, the DNS can point to a different server and we treat that each as a separate account.
To start, you could also monitor a single tenant's subdomain, and assume the others will function as long as the main one works. But, if there's a DNS issue on one of the subdomains, you wouldn't know.