Save Tooth Enamel with Onlay Instead of Crown. Why Weaker Than Full Crown? SAME Core Build-up As for a Crown? (photo)

From the pictures I am looking at, if you are asking for the restoration of choice for the seriously decayed and seriously broken down bicuspid, it would be a root canal, a post and a core build up and a full coverage crown.  PERIOD. An onlay would not be a choice here.  after all the decay is removed, there would not be that much enamel left to preserve.  The little bit that would be left next to an onlay WILL BREAK under chewing forces.  Guaranteed

After root canal treatment, post and core recommended.  Considering how short the tooth is and how much natural enamel in good shape, full crown restoration would be my recommendation for best treatment. 

Could you do an onlay instead of a full crown?  Yes, of course, but because of the severe loss of foundation and lack of solid, dependable tooth structure you are much more likely to have the onlay fail on you. 

Seeing your xray, i feel that lot of tooth structure is lost and after doing root canal a post may have to be placed for retention of the crown/onlay. The decision of either crown or onlay, can be made after your RC is done. The remaining tooth structure is evaluated by the dentist clinically and then he/she will decide what is best for you.