"OK yah" means: A British upper-class affirmation — a way of saying yes or indicating agreement, associated with the Sloane Ranger and yuppie social sets of.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.