Waiting until on board to book shore excursions so can use onboard cred: good or bad?

I'm going on my first cruise with my mom this June on Oceania from Athens to Rome. Our itinerary is very interesting and includes ports in Alexandria, Cairo (Port Said), and Jerusalem (Haifa). There will be total of 9 days at the various ports and my mom and I are interested in booking excursions for each day. My mom feels more secure and safe booking the shore excursions with Oceania (rather than independent tours), so I'm looking into package deals such as buying a minimum of 6 excursions for 25% off. Another possibility is using the Olife Choice and getting 4 free excursions. If I don't choose the 4 free excursion promotion, Olife Choice also gives the option of receiving $400 of onboard credit each. Since I'm interested in many Oceania Choice tours, which are excluded from the 4 free excursion promotion, I calculated that going with the 6-for-25%-off promotion is a better deal IF I'm able to buy the remaining 3 shore excursions on board using the $800 ($400 each person) onboard credit. My only concern is that tours will be booked by the time we board the ship. I will, of course, try to pre-book the ones that will predictably be in high demand (e.g. Visiting the Pyramids/Nile, Food and wine tour in Sicily), but I'm worried that less popular tours might also be full. I know that the availability of excursions will vary from cruise to cruise and possibly be unpredictable, but any opinions about general trends would be useful. Any helpful comments about booking shore excursions in general are welcome, as well!

Sorry for the complicated question! I truly appreciate the help!

Welcome to Cruise Critic!

You already understand the major problem with booking onboard is that the excursion you want may be full (I have had that happen). On some cruise lines the cost is a few dollars more if you book onboard rather than in advance (have not been on O so I don't know what their policy is).

For answers about personal experience with O's excursions, I would try posting your question again on the general Oceania forum.

http://boards.cruisecritic.com/forumdisplay.php?f=51

Thanks for the reply! My main concern is that there will be no more excursions left at all at a particular port rather than a specific excursion selling out.

Nvm! Problem was resolved. Turns out I was allowed to use the 4 free excursions in conjunction with the 25% off promotion!