What will it be like before and after my septoplasty for a perforated septum? And will I have to be on oxygen after plus will I have to spend the nigh

Congested. Septal perforations sometimes require silastic splints for a week. This can be uncomfortable and block the nose. Be prepared for a good amount of bloody nasal drainage for a few days. You don't necessarily need oxygen, depends on your situation. Most people go home the same day unless there are specific circumstances that apply in your case.
Usually not bad. This recovery will depend on the type of surgery which can be from relatively simple to extensive . Your recoveryay depend on pre operative health and associated problems such as sleep apnea but will not typically oxygen nor will itrequire an overnight stay. However it really is best to ask your surgeon these questions.