From Tokyo, take the Odakyu Line from Shinjuku Station to Isehara Station (about 60 min.). No need to take the Romance Car express train. The "Tanzawa-Oyama Free Pass A" is a good deal for a roundtrip, including bus and the cable car on Mt. Oyam

Valid for 2 days and costs ¥2,520. If you prefer not to take the cable car and hike instead, buy "Tanzawa-Oyama Free Pass B" which is cheaper at ¥1,560. You can buy the pass at any Odakyu Line train station like Shinjuku, Machida, Yamato, and Isehara Stations. Use the ticket vending machine.