There are many who use Omnifocus who have an Android phone now but would prefer not to give up their Omnifocus. They could easily switch to an alternate but they prefer their Omnifocus. Running a task manager without it running on the mobile is certainly possible but it is counter-productive.
For those that want to run Omnifocus on their mobile, although Omni does not create an Android mobile version, a major shortcoming of this product, there is an extremely good third party product produced by Rainer Burgstaller called Focus GTD. It doesn’t work quite the same as the IOS version but in some ways it is better if not even more logical.
Focus GTD runs on Android and syncs perfectly with Omni’s sync server. It is an excellent product that has never failed me once. Only a couple of times did sync seem to take a while but that was more near the beginning of usage rather than down the road.
Omni sync itself, no matter which product uses it, leaves a lot to be desired. As an example, Todoist, TickTick, 2do etc sync immediately whereas this is not the case with Omnifocus and you might have to even run sync manually to ensure everything is in sync. Such should not be the case in this day and age but it is.
There is no downside to running Focus GTD, if you really want to stick with Omnifocus. It is fast and more intuitive than the native version though unless you’re really accustomed to Omnifocus intuitive is not the word you would use to describe the product.
Regardless, Omnifocus is a comprehensive package and those with a lot vested in Omnifocus may not want to give it up simply because they have an Android phone. If this is you, simply go the Google play store and look for the product. Once you’re up and running on your mobile again you’ll be much happier.