Google holds all the cards. They have to sign off, or the phones will lose access to the Play Store and Google services like Maps. Google only needs to play the cards it already holds.
The SoC / OEMs also have to sign off because they are the ones with access to the driver source code which needs to be updated and recompiled against a new release.
If Google plays hardball, these parties can always threaten Google with "We're leaving the OHA; we will be going with Amazon's Android fork and bundle Kindle and Prime apps." I'm sure Bezos will throw in Amazon Maps for free.