OSPF Hello packets unicast and multicast
On a broadcast network, as soon as OSPF is enabled on a router, it starts sending multicast hello packets to the well-known multicast address of 224.0.0.5. When these are received by an OSPF router, it responds with unicast hello packets. Subsequent exchange of hello packets takes place using unicast communication in both directions.
Links:
https://networklessons.com/ospf/ospf-packets-and-neighbor-discovery