Who are responsible for pushing out the OTA updates? There are many parties involved in an Android phone. My question is who is responsible for the OTA updates of my phone. Is it the manufacturer, the phone service provider or Google?

I thought this answer could be answered generally but maybe not. My phone is the Galaxy Nexus i9250 witch is manufactured by Samsung.
If your handset is Google Nexus, then yes, Google pushes out the OTA updates.

The rest is down to the carriers, depending on who you are with, what handset you have.

Edit: Added more thought for clarity.

Some may not roll out the OTA updates, its all dependent on the market, region. I am with O2, and never get any OTA updates what-so-ever. Some manufacturers do include a 'PC companion' suite that accompanies the handset  and it is through that pc companion suite, that there may be a "firmware" update that is independant of the carrier used. That would be an exception to the rule, although the trend seems to be, to "use the official manufacturer's handset companion software for Windows" and to check on that regularly.