Yes with OVO your SMS and MMS are unlimited too - as long as they are to being sent to another Australian phone. With OVO Mobile - Medium, Large and Extra Large you can get unlimited standard calling, SMS and MMS within Australia - it's just part of the package. If you've chosen OVO Mobile - Mini or OVO Mobile - Small you've signed up to a plan with a calling allowance so that you can make standard calls.
If you want to call, SMS or MMS to a phone outside of Australia all you need is OVO Extras. OVO Extras is a small amount of credit that can be added to OVO for Mobile. Or, if you're going overseas you'll need to buy OVO Roaming to add a small amount of credit so that you can roam abroad.