Obfsproxy is a protocol that only bridges use, and only a subset of them, so there are no entry guards involved. By using obfsproxy, you are ostensibly already hiding your Tor use because you are using a bridge. Well, you are hiding it from IP address scanners, but the connection can still be DPIed, which is what obfsproxy is for. You have to manually select obfs2 or obfs3 bridges from the BridgeDB (https://bridges.torproject.org) to use obfsproxy. You also need a special browser bundle: https://www.torproject.org/docs/pluggable-transports.html.en#download