While you can set the cost to fix you immediate problem, there are really only two long term solutions to managing traffic that we tend to use when designing Service Provider networks.
1) BGP - OSPF isn't designed to influence the path of subnets using policy, BGP is designed to do exactly that
2) MPLS Traffc Engineering - This does require a fair amount of complexity to add to the network, but it allows you to manage the exact amount of traffic along a path of next hops that you can specify - regardless of what the routing protocol says.
http://wiki.mikrotik.com/wiki/Manual:Routing/BGP
http://wiki.mikrotik.com/wiki/Manual:Simple_TE