There's no outward physical signs of castration with male cats - even a day after the op you'd be hard pressed to see any difference. I would assume therefore that his neutering is fairly recent as Jez is being territorial, Oscar will still have some male urges which are making her uncomfortable.
For this reason alone they need a slow introduction. Oscar's urges to wander and mark territory may still be with him and it can take up to six months for the hormones in an adult tom to settle and for him to relax into a life of contented celibacy. Not that you need to keep them apart for six months - a week or two should do it, but slowly slowly when you do start letting them spend time together, and never unaccompanied until you know for sure they won't fight.
The best thing you can do to relax the situation in your household, is to relax yourself.
Seriously if you are stressed, the animals will pick up on this stress too. I'm sorry you've found yourself on your own but try to relax around your animals and enjoy your new freedom, and use your new family member as therapy for your own anxiety.
Now you have Oscar who is blind you have someone who will always want to hear the sound of your voice
so talk to him, go sit with him and tell him all about yourself, let him know how much he means to you, y'know small talk? He'll be your adoring best friend in no time.
You don't need to hold back, cats are unconditional once they've decided to love you.