Indirect questions

In your opinion, why was there a need for the OECD to design tax action plan to reduce tax avoidance?"

(use 'was" if the tax action plan has already been designed)

Liza, my immediate reaction:

...., was there the (?) need for OECD to design a tax-action plan....?

For tongue untying you could use 'a' or even delete altogether: '...was there need...'

'tax action' is presumably meant as a compound adj and therefore the need of a hyphen