Antwerp (ANR) could be considered to be a similar airport to Ostend, the difference being that it can't handle jets. It can only support flights to Manchester and London City.
Belgium is a small country with a very efficient rail network. You can be in Bruges, or actually most places in Belgium about two hours after leaving Zaventem.
Charleroi has developed its own niche handling the low-cost carriers. Even though it has not got a station, you can be in Brussels in forty minutes. It is worth noting that even Ryanair couldn't make STN work to CRL.
The other notable airport in Belgium is Liege, in the French-speaking Wallonia. It is mostly a cargo airport, with holiday flights. Despite being Belgium's third city and being extremely close to Maastricht in Netherlands and Aachen in Germany, as far as I know it has never attracted flights from Britain or other parts of Europe.