I found few instances of「プール行くかオイ」in the manga I'm reading like this one below.
It seems to means「プール行こう」but I don't understand the purpose of「かオイ」sentence ending. I thought at first that this オイ is an injection meaning "hey" but I don't think it makes sense in this context. The speaker of this phrase is a woman and it's weird to me that it is placed at the end of the sentence, not the beginning.
Is「かオイ」some kind of slang?
It is only the ending か + オイ ("Hey!"). With オイ, it sounds like the speaker demanding the listener's attention/response.
This sounds rude, and in that sense, can be called a slang. Depending on contexts, it can mean hostility.
Note that オイ is not a grammatical ending. It is just Hey. プール行くかオイ : Are we going to a pool, aren't we?
can be written プール行くか、オイ or オイ、プール行くか.
The fact that this is said by a woman or a girl is not that surprising. I think that 'strong' girls often talk that way in manga, even though very few girls (and possibly no women of decent age...) would do so in reality.
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A particular phrase that comes to my mind is やんのかオイ, which means something like Are you gonna fight with me?. This オイ can be replaced with コラ (やんのかコラ). These are typically used by naughty boys (不良) in manga confronting with each other.
