It is a bit difficult to define ojas as per modern methodology. Ojas is balam = strength. Body complexion, voice etc. are the outcome of the ojas. You must be knowing 7 dhatus (tissues) in Ayurveda. The essence of these dhatus is ojas (if am not wrong). It can't be measurable. Understanding ojas is as good as understanding tridoshas.

You may get reference from Caraka Samhita, Cikitsa Sthana (6) - 8.24. Under this reference, the ojas is described as 'deha saara' (essence of body). At 6.15.3, ojas is defined as - ओजः हृदयस्थं सर्वधातुसाररूपम् । (sentence taken from ChakrapaaNIaym on Caraka Samhita) This expression attests my earlier statement. Here, the seat of ojas is also defined - which is the heart.

By the way, I am not an Ayurvedic doctor. But a Sanskrit faculty working with B.A.M.S./M.D.(Ayu.) students for their PhD degrees.

A very good Question. Today there is much talk on "Wellness". Yet, there are no markers for it. Absence of high cholesterol or Blood sugar does not automatically mean one is "well". I still think there is a way to measure Ojas. We have to think deeper.

Ojas is the ultimate product of food digestion. Food gets digested and forms the sapta dhatus, seven tissues, sequentially. The final tissue is the reproductive tissue, Shukra. If utilised for reproduction, Shukra does not produce ojas. Conserving Shukra produces Ojas, giving immunity, health and also lustre to the skin.

Regards

Biochemically, there are 1000's of digestion products. So, the statement that Ojas is  "ultimate product of food digestion" is vague. ..but many have stated this. Perhaps, Ojas means fertility..because great physiological and mental health is needed in order to produce good quality and quantity of sperm and eggs. If one is very fertile, it means one is very healthy !  Ayurveda has a great way of getting to the relevant phenotype!

Exactly which physical body component is oja is not well understood in modern or physiological language but references in ayurveda indicate more than one entity. Oja is read in context of Rasadhatu(ch. Nidansthan4).It is also used in reference of prana(shushruta sutra sthan15).Another reference is for excellence of dhatu (shushruta sutra thanks 15). Many references are there for ojas. Also referred as Bala. If it is assumed imaginary,types of oja,physical properties of oja and such other points suggest that it must be a physically existing entity which functions for harmony for structural body components with their functions and also with mind.