Feeder schools are a bit confusing. As I understand it priority is given to children in catchment, regardless of which school they go to. If a child lives outside catchment, and goes to a feeder primary, they will be next on the list - before other out of catchment children. That said - Swansea has lots of secondary places - neither Olchfa, Bishop Gore nor Bishopston were oversubscribed last year, so it wasn't relevant anyway.
However, the schools do have very good transition links with their feeder schools, which means that there is often a fair amount of contact between the "usual" primaries and secondaries.
Olchfa is seen as the more "academic" of the schools, with a generally very "well heeled" parent. Bishopston is more rural, and again is in a reasonably upmarket area, but has no sixth form. Bishop Gore has a far more mixed cohort, which does affect their published results.
I chose to send my children to Bishop Gore, and I am very pleased at the decision we made. I have three very different children,and they have all settled in there really well. My eldest only went in the Sixth Form and has significant SEN. He was accepted by his peers easily and almost all the staff went out of their way to support him academically. My daughter is very bright and very involved in sports/singing/drama at the school. She is being challenged academically and their performing arts department are extremely talented teachers, and they put on amazing shows. My youngest is also a bright spark, doing well in lessons and enjoying himself. He is a regular at Geek Club, got into the finals of a computer programming competition recently and is making a music video in Visual Media club.... so yeah, they are happy, so I am too.
Anyway - officially Brynmill, Whitestone, The Grange and Oystermouth are feeder schools for Bishop Gore. Olchfa has Killay, Hendrefoiland and Dunvant. Sketty and Parklands feed into both schools, depending on your home address. But as I say - this is by no means a dictat on where your children will go - you can apply wherever you want, and a lot of children go outside catchment.