Microsoft OWA 2013 Response header Truncation

It's not clear to me what you mean by truncate the header. If you are talking about removing HTTP headers or masking header content, that's straight forward and documented with myriad examples on the iRule wiki: https://devcentral.f5.com/wiki/iRules.HTTP__header.ashx.

That being said, I'm pretty sure you can configure IIS (or whatever Microsoft fronts OWA with) and/or Exchange to not send out server information. If that's not possible that would mean MS sucks; Which they do, but I'm not sure they suck on this particular issue.