When will ECG be available in Oman?

maybe never

ECG is viewed as medical equipment and is therefor illegal for apple to introduce in your country, without the approval of your government.

Thanks for asking about the new ECG feature.

Check here for availability in your region: ECG

ECG requires the current non-beta version of iOS and watchOS. For additional help with the ECG feature on your Apple Watch, take a look at these articles.

Heart rate notifications on your Apple Watch - Apple Support

Taking an ECG with the ECG app on Apple Watch Series 4 or later - Apple Support

If you have any other questions please reach out to our team here: https://apple.co/HeartFeatures. If you believe that you are experiencing a medical emergency, please dial your local emergency services.

Take care 

For the ECG app to work on your watch the ECG app has to be approved by;

1. the government of the country in which the watch was bought and

2.the government of the country in which you reside.

Yes it will be your government in Oman that deals with the health system.