According to one answer in the Ramban it comes out that Noach’s daughter’s-in-law were children of the giants. This is another explanation how the giant race survived the Mabul.
According to the other answer that Og himself survived, the Navi in Yehoshua explains that all the giants in Eretz Yisrael were defeated by Yehoshua and his army. The only four remaining giants lived in the land of the Plishtim. They were Goliass and his three brothers. Dovid and his army killed them in later years. This is why there are no giants today. I can look again for the sources for all this if anyone is interested.