OMEMO conversations work in Monal. There is still a lot work to be done here to properly handle keys and make UI elements in OSX and iOS for all the the new interactions. There is a lot that also needs to be tested before this is in ready for the App Store. Realistically, expect to start seeing this stuff in the binaries I release some point in August.
I have tested this with gajim on linux and chat secure on iOS. Will have dust off an android device somewhere to try it with Conversations.
I have released 3.0.2 to the iOS App Store. So far I appear to have resolved the worst crashes.
I have an update on the status of OMEMO in Monal. I’ve completed my spike and have a very rough implementation working. I am able to communicate with Gajim and Chatsecure. I am actually using a lot of the same OMEMO code as Chatsecure using Chris’ cocoapods. The shared code base should reduce duplicated effort and ensure compatibility on the two main Apple platform clients going forward.
The current code isn’t anywhere near production but once I clean it up more, you should start seeing it as an option to turn on in Mac betas in the next month or so. Below you can see my interactions with Gajim and Chatsecure. 
I am still cleaning up all of the issues people have seen (and some old friends) in the latest releases. There are new betas out. I will looking for feedback and crash reports. I hope to have the next updates out this week. I know it has been almost weekly releases since the 3.0 release. Hoping to slow down to a more manageable release cycle after the code is more stable.