Hi! I got my hair straightened twice [ chemical straightening ] , and I plan to get it done again.. I just wanted to know whether its harmful for my hair ? Or will regular oiling help?

It is quite damaging to the hair as the solution applied to the hair penetrates into the hair and reshapes the hair.
If you want to get it done, go for it! Just as long as your hair is in good condition. Hot oil treatments or just normal conditioning treatments help to keep the hair in good condition.