How can I tell mikrotik (2.8.11) not to announce routing to private nets (10.0.x.x) to other routers via OSPF?

you can't do it.

How can I tell mikrotik (2.8.11) not to announce routing to private
nets (10.0.x.x) to other routers via OSPF?

if you're redistributing routes either static or connected, you cant filter anything in or out. If you're using only intra-routes (connected routes), you could do filtering with "Network" by excluding the unwanted subnet (e.g. 10.x) from the networks included in OSPF.

Shpendi