Does the prayer become void if the worshiper forgets to say the Takbir when standing up from the first Tashahhud, then remembers it only while reciting Al-Fatiha, so he interrupts it, says the Takbir, and then resumes reciting it?

You were obligated to continue your prayer and not return to the takbir. Your return was an error, but it does not invalidate the prayer. Some sources have mentioned that interrupting a verbal pillar of a supererogatory prayer, such as interrupting the Fatiha to begin another rak'ah, does not invalidate the prayer at all, because it does not alter its system or apparent form. Your uttering of the takbir during the Fatiha is considered a permissible dhikr but in the wrong place, and it necessitates the prostration of forgetfulness (sujud (definition) al-sahw) according to some scholars, though omitting it does not invalidate the prayer. Note that saying the takbir when rising from the first tashahhud (definition) is a Sunnah (definition) according to the majority of scholars, and the prayer is not invalidated by intentionally omitting it. As for your saying "oh" out of dominance (overcome by it), it also does not invalidate your prayer. Based on the foregoing, your prayer is valid, and you are not required to repeat it.