Drive along Via del Teatro Romano, then keep straight along Via S. Spiridione, straight along Via Filzi, then turn right into Via Milano. Keep driving straight along Via Milano and Via del Coroneo. At the end of Via del Coroneo, turn right into Via Fabio Severo. Keep driving along Via Fabio Severo, then along Via Alfonso Valerio. At the end of Via Alfonso Valerio, turn left into Strada Nuova per Opicina. Drive along Strada nuova per Opicina, then along Via Nazionale. At the roundabout, take the second exit into Via Nazionale. At the roundabout at the end of Via Nazionale, take the first exit into Strada per Vienna, until you reach Località Fernetti. You need to buy the vignette (road tax) in order to drive on motorways in Slovenia.
To avoid paying the vignette, once you reach Località Fernetti you can turn right towards Sezana, then pass through the town and then keep driving towards Senozece, Razdrto and Postojna.
It takes about forty minutes by car from Opicina to Postojna; if you keep going you can reach Ljubljana too.
Please note that road signs in Slovenia are yellow for regional roads, green or blue for toll roads (where buying and displaying a vignette is compulsory).