This is not at all a trivial task to do and comes with so many implications and detailed questions that will have to be thought about that I'm unsure one can do this meaningfully in any way.

For a POS it is relatively straight forward. Someone shows up at your counter while you just happen to lose connection to the internet. Your customer usually holds the goods to sell in their hands, so there is rather no point in checking actual stock availability. 

For a purchase workflow (PO) this might be different: do you even need to purchase it? Did anyone else purchase it in the meanwhile already? Do we need to merge these POs or is one obsolete then? 
Again, these questions are irrelevant to a POS use case as no customer would ever go to the counter twice just to buy the same item s/he just paid for.

In any case, what you would have to do is, upon every relevant create, write, unlink operation in Odoo, store all data relevant to an offline process to the users session. The local storage for each user will obviously get quite massive. Also, you will have to consider new users: upon their first connection ALL data relevant to your offline process would need to be fetched.

Once and if this is set up, next issue will be the rendering of the data. You're currently offline so there is no chance of having the server render your list and form views needed to actually visualize the data.

Then, if in any case this topic would be sorted, you need to handle local changes, store them and push them to the server once your online again.
Again, for a POS this would just be a limited set of views not even requiring any sort of high-end navigation or menu system. Also, for example, without a server, how are we going to deal with rather simple things such as a computed field? What do we do with SQL constraints that start to act up due to having the same thing written once online and offline?

Now, this is just been a great simplification of steps and things involved, but those few lines in my opinion already highlight the massive load of work you will have to deal with.

That being said: consider alternative approaches.

For example, could your staff run a local Odoo server on their notebook that is in constant - when online - communication with the actual server? This way you could try to run delta syncs from one Odoo to the other. Still a massive task but at least you don't have to deal actually visualizing data when offline because your locally installed Odoo will just work as well without any internet connection.

Another approach can be a custom made mobile app, if it's strictly necessary to support these devices, with a simplified, in-build rendering mechanic (=views defined in the mobile app) that allow for specific operations meant to cover offline scenarios.