Ojas is both the gateway and the container. It is the gateway between consciousness and physiology, between spiritual and material. As the subtlest physical, it is a container. It contains the flow of cosmic energy (Prana) from Atman through the individual.
Ojas is the first material expression of consciousness. It is both manifest and unmanifest. It pervades the body, has a yellowish-red color, looks like ghee, smells like puffed rice, tastes like honey. It is similar to balanced Kapha; only not as heavy.
Ojas is Primal vigor. Ojas is the subtle energy of water, the stored-up vital reserve, the basis for physical and mental strength and endurance. It is to Vata, Pitta, Kapha, as Samhita is to rishi, devata, and chhandas. It holds the body together. It allows the body-mind’s balanced functioning. Ojas is responsible for maintaining contentment, luster, strength, and health. It is the main factor responsible for immunity.
Ojas is produced at every stage of digestion and is the finest end product of digestion. As such, it is the essence of the body tissues (dhatus).
There are 5 factors which increase Ojas: behavioral Rasayanas, devotion (Bhakti Yoga), medicinal Rasayanas (includes Ojas increasing foods), fresh, wholesome diet, 5 ways to eliminate Ama.
There are 7 factors which hamper the flow of transformation of the Dhatus causing Ojas to be less or imbalanced: excessive injury/harm to the body; excessive loss of substance; overexertion; excessive anger; excessive grief; excessive thinking, worry, anxiety; excessive hunger.
http://en.wikipedia.org/wiki/Ojas