Hi, i am upgrading small wifi bridged lan to OSPF routing with two ISP For now i have done load balancing with interface costs
Users from R2 -> R1->ISP1
Users from R3 & R4 ->R5 ->ISP2
And works failower if any links broken.
Problem is R2 serves ~60 clients R3+R4 ~20
And i want that some users From R2 (example 172.17.1.0/27 to be routed R3->R4->R5I->SP2; all others by default R1->ISP1
how its possible without breaking failower?

Two things that come to mind right away are

1) Static routes with check gateway enabled. You could also couple this with NetWatch for more advanced failure scenarios.

2) BGP - OPSF starts to break down when you try to move traffic by subnet. BGP is much better suited to this task.

I need help to understand how to use BGP in my example
for iBGP Area must be full mesh connections, right? so i need different area for every single router?
How BGP will know that any link is broken? do i need special scripts and so on?
And for my task how can i filter needed default route not best path and mark routing for dedicated subnet?

This is extremely pleasant and conventional post...You shook posting it....Thanks a considerable measure for posting it....!!!

Here is a link to a presentation we did on BGP at USA MUM 2013. This will give you a good idea of how to build a basic BGP network. Route Reflectors can be used to provide full mesh.

http://mum.mikrotik.com/presentations/US13/kevin.pdf

Fantastic it is exactly what i needed!!! Thank you IPANet!
in future i will post example of configuration from my network for other newcomers to routing world

Glad we could help you out. Good luck!!