Why should project managers complete hard problems first in an OOSAD project?
Ans) The query really holds good in general for each situation in life. It is one of the principles of good time management.
The idea is to tackle hard (and significant) problems first. This, if resolved - will pep up your confidence to deal with other not so hard issues. Also, this could have cascading effect on other issues that might get resolved on its own.
I would rather stress on "significant" than "hard" issues. If a "hard" problem is not coming in the way of your deliverables (means it is not significant) - keep it aside. There is no require to spend a lot of time on it.