Suggest 4-wheel drive auto & GPS.

If you wish to avoid the 3.5 hour drive ahead of you, you may consider a Sansa or Nature Air flight from San Jose to Palma Sur, arranging in advance to be met by a car rental agency at the Palm Sur Airport. The drive from the Palma Sur airport to Osa Mountain Village is about 45 minutes. The flight is about 30 - 45 minutes.

Driving: San Jose to Jaco takes about 70-90 minutes, via the Costa Highway (Hwy 34). The speed limit is 90kph in most areas. From San Jose to Jaco is 72 miles. From Jaco to Dominical is another 74 miles.
DRIVING THE COAST ROUTE
If you have a GPS, I highly suggest setting it at the rental location to take you to Dominical.
From the airport, take the highway into San Jose and you will come up on Sabana Park on the right side. Take the highway to Escazu and Santa Ana (there will be a sign to direct you.) You'll continue on the highway following signs leading you to Jaco. You'll get on the Costanera Sur and continue following signs toward Manuel Antonio/Quepos (Stay on highway, don't exit to Manuel Antonio). Continue south past Dominical. OSA MOUNTAIN VILLAGE is located on a 45 minute drive South from Dominical. Then past Uvita (great spot to stop at a local grocery store and stock up on food and alcohol). We are between KM markers 196 and 197 on the coastal hwy (Sign on left), take the dirt road on the left (East) side of the road between the mile markers. It is just about 2 miles past the new hospital. If you get to the Osa Mountain Canopy tour office on the right hand side, you have gone too far. If you reach Palmar Norte, you have gone way too far…. At the turn-off between km markers 196 & 197, take a left off and then follow the signs up the mountain (it is quite a ways up the dirt road; you should take a left, go a ways and go over a bridge then go a ways and take a right where it will start to get very steep, you will go a long way and the views will get stunning, keep going until you get to a fountain/sculpture, at the fountain you will enter under the security arm and follow the road down the curve to the pool area where you will check-in for your stay.)