Usually, you can tie it to "OKAY" all the time, if:
1): The slaves on the APB bus will never generate "error" response (that means every r/w transfer will success). This is usually the digital circuit does.
2): The slaves on the APB bus can response the r/w access quickly enough to met your AHB bus requirement. (this means there is no bus efficient issue, so there is no need for you to use "RETRY" and "SPLIT")