all the necessary info should be on the 1099-B you should get from the broker
the spinoff of OGN was a nontaxable transaction.
you need to allocate the cost of the MRK between the OGN and MRK. the broker will probably do it for you
here's what was reported to te iRS
https://www.merck.com/wp-content/uploads/sites/5/2021/06/Form-8937.pdf#:~:text=For%20example%2C%20if...
the method suggested would allocate 4.77% of the cost of the MRK to OGN
with the remaining 95.23% staying with MRK
the holding period for OGN is the same as the holding period for MRK because it was a non-taxable transaction
