My dad passed away a few months ago and I have his 1400$ stimulus check. How can I go about getting it cashed?

If the check was issued after your father died, I would assume (without knowing all the details of what the check was for) that nobody has he right to cash it.  If it was issued before he died, it now belongs to his estate, and only a duly appointed representative of the estate can cash it.  You would have to start a court proceeding to probate his will, or if no will to administer his estate, the Court will appoint an executor (if one is named in the will) or administrator if no will (they may use different terms in Ok, but same basic procedure), probably you if you want to do it, and that person would then take care of marshaling and distributing the assets of the estate, including this check.