grpc-okhttp works fine on regular, non-Android Java versions. Security is different than grpc-netty, so you can't use netty-tcnative. You can use Java 9+ or Conscrypt. If using Conscrypt, you should install it as the default provider with
Security.insertProviderAt(Conscrypt.newProvider(), 1); before calling grpc. Netty uses non-blocking I/O via NIO whereas OkHttp uses blocking I/O with Input/OutputStreams. Thus each TCP connection with OkHttp will use 1-2 threads.
Why are you interested in using okhttp with non-Android?