We bought OBB tickets on 7/13/21 and then immediately (that same day) got park tickets and park reservations. Trip was the first week of October.
My e-mail also tells me that getting OBB tickets must have put me in a
Disneyland mood, because just two days later, on 7/15, I was getting tickets and making park reservations for a Dapper Day trip in mid-November.
So park reservations were open at 120 days. But even though October probably had been open for awhile, we had no trouble reserving for our dates once we got OBB settled.