You'll need some top-mounts.
When the Ohlins coilovers are removed, all parts, with the exception of top mounts (beacuse you don't have these), should be available to you to fit the new OEM struts.
The 3 bolts that bolt in the bay will be usable with OEM top mounts, which are probably cheaper to source second hand than they are new... I believe they are quite expensive. The 2 large bolts (one of which might be a camber bolt) that hold the base of the towers to the hub should currently be securing your Ohlins; these will do the same job on an OEM unit.
When you sell the Ohlins do no sell them with any bolts, as you will need all the nuts and bolts that secure the Ohlins to the car will do the same for the OEM suspension.
In the politest way possible, if you're not familar with suspension, enlist the help of someone who is, or take it somewhere; If you're not sure how it goes together and with what parts, then it's not a good idea to take it apart - as your query kinda suggests.
Otherwise, good luck, work slowly and carefully.
You're removing old components, copious amounts of WD40, patience, strenght, and care, need only apply.