"When was the last time you saw him?" [STRIKE]or[/STRIKE] and "When was the last time you did it?". Are these correct (no question mark here) or should the first form be used for "saw" and "did"?
Note my corrections to your punctuation above. "Or" was inappropriate. You weren't asking us to choose between those two sentences. You gave two separate examples for our consideration so you should have used "and".
I don't understand your question. What do you mean by "first form"? You used "saw" and "did". What alternatives are you suggesting?
I am not familiar with the terms "first form" and "other form". But I can assure you that "when was the last time you saw him" and "when was the last time you did it" are perfectly natural. No native speaker could be in any doubt about their meaning.