okhttp application level OkHttpClient instance

Using single instance is not a problem instead it is a common practice. You can check a similar app from github which uses dagger to make OkHttpClient singleton and injects it other modules.

And you can see in this discussion JakeWharton is also suggesting this kind of usage.

But it is better if you use a Singleton Pattern for this purpose.

Apart from what @bhdrkn correctly suggests, to definitely confirm that a singleton instance of OkHttpClient is a correct way to go, an excerpt from documentation:

Source: https://square.github.io/okhttp/3.x/okhttp/okhttp3/OkHttpClient.html

OkHttpClients should be shared

OkHttp performs best when you create a single OkHttpClient instance and reuse it for all of your HTTP calls. This is because each client holds its own connection pool and thread pools. Reusing connections and threads reduces latency and saves memory. Conversely, creating a client for each request wastes resources on idle pools.

Refer to Javadoc (link above) to see correct ways of initializing OkHttpClient instance.