The best way to get to the Orosí Valley is by taking the bus that departs in Cartago, a city just east from San José. From here it takes about 45 minutes to get to the village in the middle of the valley.
Just south-west of the Church Ruins in Cartago you can catch a bus going to Orosí. It is a flashy green bus, with line 339 on it. You can pay for your ticket in the bus: it costs about 1 dollar to get there.
When you want to get back, take the bus at one of the few busstops in the centre of the village. Busses in Costa Rica are not the same as in the rest of Central America: here they don't stop just anywhere when they see passengers, they only stop at the busstops.
I came to Orosi directly from San José by local bus. It's a very cheap way to get there but a little difficult to navigate even if you know Spanish. Mind you, this is not gringo infested like many other parts of CR, so not that many people I talked to knew English. Luckily I know Spanish, but that didn't really prevent me from being utterly confused. If you're really uncomfortable with this, take a taxi from San José.
You need to change buses in the city of Cartago and the transfer point is a bit unclear. I would have been in a lot better condition had I asked the bus driver while boarding to tell me where to get off. I figured I could find the transfer point easily on my own, but boy I was wrong. I got off in Cartago on the opposite side of town. Luckily my bus driver was sweet enough that he turned around and drove me personally back to where I needed to be. Talk about great friendliness!
Here's the directions:
-Take the directo bus from the Cartago bus terminal in San José
-The terminal is on Calle 5 and Avenida 18
-You line up and board a bus when it arrives; it leaves when it is full.
Buses are frequent, every 10 minutes or so.
-Get off the bus in Cartago “cerca de las ruinas” and walk to the Orosi bus stop on Calle 6 and Avenida 1.
-This bus leaves hourly, on the hour, on weekdays and every half hour on weekends.
-You will first pass through Paraíso. Once you descend down a winding road into a valley, you know you have arrived in Orosi. You will first see a sports field on your right. Soon thereafter are the main stores in town. Get off the bus anywhere along this road.