The crux of Walking in Zen, Sitting in Zen by Osho is the concept that every moment can be a meditative experience. Osho teaches that Zen is not confined to the formal practice of sitting meditation (zazen) but extends to all aspects of life, including walking, eating, and even breathing. Through mindfulness and total presence in each action, individuals can experience a deeper connection with themselves and the world.

Key themes include:

1. Mindfulness in Everyday Life: Osho emphasizes that Zen is about being fully present in whatever you are doing. Whether sitting, walking, or engaging in daily tasks, if done with complete awareness, every moment becomes a form of meditation.
2. Simplicity and Spontaneity: The book highlights that Zen is not about following rigid rules or techniques but about embracing simplicity and spontaneity. The more one can let go of intellectualizing or controlling, the more the mind can become still and naturally at peace.
3. Unity of Body and Mind: Osho speaks about the integration of body and mind in Zen practice. The way you walk or sit can reflect the state of your mind. By cultivating awareness in simple actions, you can harmonize both, leading to a deeper sense of inner balance and tranquility.
4. Living in the Present Moment: A central tenet of Zen is living in the "here and now." Osho encourages readers to drop past regrets and future anxieties and fully immerse themselves in the present moment, where true peace and understanding lie.

In essence, Walking in Zen, Sitting in Zen is about seeing every aspect of life as an opportunity to practice mindfulness, where simple actions, when done with awareness, lead to spiritual insight and inner stillness.