Interestingly, what we found through our research was that nobody could agree on a definition of "application" in a microservices world. The way OAM is designed currently doesn't enforce a rigid "application" structure for services. We have a concept of application scopes that can be used to place application-like boundaries around groups of services (modeled as "components" in OAM). For example, grouping services in a "health" scope where the health of each service in the group is evaluated when any one of the services is upgraded as a trigger for automated rollback is something application scopes are designed for.
disclosure: am one of the OAM authors.