I'm using Office 365 small and want to know if I can use Visual Studio for Event Receiver or Timer Job?
You can create a "remote event receiver", where O365 calls your service that is hosted on your machine when an event occurs. So, if an announcement is added to a list, for example, O365 would call a WCF service that you registered for that event. That WCF service could be running on an IIS box in your own data center. The only difficulty with this is that MS never got around to documenting how to actually set it all up...
Timer jobs are also not supported, but you could create a powershell script that runs on your own server that accesses O365 via the client object model, and you could schedule that script to run via the task scheduler.
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1Kirk Evans at MS has an example on how to run a "TimerJob" as an app: blogs.msdn.com/b/kaevans/archive/2014/03/02/… – Anders Aune Nov 4 '14 at 6:19