the correct form is donor as in a blood or organ donor.

A deceased donor, or simply an organ donor. They used to be referred to as a cadaver donor but that term has fallen out of favor.

Group 15 compounds such as Arsenic, antimony, bismuth etc. which can donate electrons

Not really...

Both "donator" and "donor" are acceptable terms, but "donor" is the more commonly used and recognized form in English. It is recommended to use "donor" for clarity and consistency.