That's a good question!
If you do want to use your phone overseas you'll need to buy OVO Roaming - it's a small amount of credit that can be added to OVO Mobile. OVO Roaming let's you make calls overseas and send messages while you're out of the country. With OVO you add OVO Roaming credit when you need it and when you don't need you're under no obligation to top it up.
Your OVO Roaming credit is separated from any other credits. From the time you top up you've got 90 days to use it up.
It's really easy to add OVO Roaming to your OVO Mobile service and your credit is available almost immediately. Just head to your OVO Dashboard to add on OVO Roaming - or if you've already got OVO Roaming set up you can use your OVO Dashboard to check how much credit you've got left