Previously best friends

I have been reading a similar thread, but thought our situation might warrant a new one.

We moved into a new house six months ago and our two cats quickly settled in with no imediate problems, they remained best friends until the last couple of months. Otter the more daring of the two now spends more time outside and has been spending some time next door with our elderly neighbour (Although she does not feed her, etc). Oscar meanwhile has stayed closer to home and is his usual overly docile self. Although Otter has remianed docile towards us (if a little less likely to sit on our laps as she used too) she has become more agressive towards Oscar, hissing and even swiping at him when he comes close. This does not occur everytime they are close, they still eat together but they no longer sit close to eachother / bump heads as they used too.

On reading previous advice, we have considered keeping them both inside for a while so there scents can match each other again beliving this might be the cause. Showering attention on Otter has relaxed her a little bit in the past weeks, the vet has proposed pheromone sprays in the house to try and relax Otter although we believe this may cause the already chilled Oscar to trip out

Any advice to help these two get along again, appreciated.

You will never see me advocate anything but an indoor-only lifestyle for cats. Besides whatever annoyance one cat brings another one from outside, there are also icky fleas, earmites and other gifts. I would say keep both of them inside all the time and invite the neighbor in to visit them.

Inside only is a good idea, and if the female isn't spayed, that is the thing to do. if neither are altered, the female may find the male unworthy of mating, or she may think this even if he is neutered but she is not spayed. If they aren't altered, get them altered! Hopefully helpfully,
joecool