Hi, I need to specify different OSPF costs for the relationships between a client router and two server routers (primary-backup) on the same layer 2 network, as the client router is on the other side of a wireless link.
I need to avoid the creation of vlans as this situation is very frequent and would mess our network.
I think cisco allows the creation of virtual interfaces (point-to-point subinterfaces) to solve this situation.
Is there any equivalent in Mikrotik?

Do the server routers connect with another link aside from the L2 segment that the client is accessed on? If so, you should be able to adjust the interface cost on the least desirable link.

If they are all on the same L2 segment only and you want to adjust cost to a single router, I'm affraid the answer is that it isn't possible within OSPF. The only way you would be able to manipulate this would be at an area border, not within a single area.

The nature of OSPF is that the LSDB (Link State Database) will be identical for all routers within a single area. If you wanted to adjust the cost so that it would only be locally significant, it would fundamentally break the way OSPF works.

You could use some static routes with check gateway or perhaps some static routes with scripts that check the route table.