iRule for directing OWA/OA Traffic

Yes, if the pool isn't changed by the iRule the default pool will be used. You could also try something like this:

when HTTP_REQUEST {
    if { [string tolower [HTTP::host]] equals "webmail.company.com" } {
        switch -glob [string tolower [HTTP::uri]] {
            "/ews/exchange.asmx" {
                pool Exchange_2013_proxy_pool
            }
            default {
                HTTP::redirect "https://site.company.com"
            }
        }  
    }
}