2. Question: Why are you allowed to add permissible wood to muktzah wood into an oven on Yom Tov as a permissible means of burning the wood and cooking with its heat but there is an opinion in the Shulchan Aruch (Orach Chaim 677:4) that if oil already used for the Chanukah menorah got mixed up less than 60 times its amount in regular oil you cannot add more oil to nullify it?
A. The Shulchan Aruch In Orach Chaim 507:2 says that one may add wood on top of muktza wood on Yom Tov to burn them in an oven because it is permissible to purposefully nullify a rabbinic prohibition which can be fixed as long as one does not get direct benefit from it when it is burning up.
B. The Chanukah oil is also rabbinic and some do permit purposefully adding more oil in order to nullify it and use it for whatever purpose you want like a light source. (See Mishna Berura 677:4:20.)
Answer: The Mishna Berura (507:2:8) says the reason why the other opinions don’t permit nullifying the oil is because in this circumstance you will be getting benefit from the light of the fire in the candle while the oil or wax is still in existence but by the wood the benefit of heating up the oven isn’t coming from the heat of the wood until after the fire burns it up.