It is possible to take a bus from Olbia to Porto Cervo, but there are only 4 connections per day. During summer the Sunlines bus makes a stop at the parking lot near the cruise terminal. The ride to Porto Cervo takes about 1h15 minutes and costs just a few euro per person (one way).
Roughly, these are the departure times from the port: 08h05, 12h40, 15h40, 18h40. The returning times from Porto Cervo are 7h50, 11h45, 16h15, 18h50
Sunlines does not have a website, but here you can find some more info.
ARST bus company also has a regular service from Olbia to Porto Cervo (line 604). The buses don't run frequent and don't stop at the port. You have to take a bus at the city center.