We have configured Office Web App for our SharePoint 2013 Farm and in Site settings, Site Collection Features, have disabled "Open Documents in Client Applications by Default" . Out of 15 sites, all documents in 13 sites open in Office Web App. One of the 2 sites where the document still open using Client Application is a Wiki site.
Advanced propertied for the libraries settings has been set to "Use the server default (Open in the browser)". Anything else to check to open documents in Office Web App?