I worked my way through a degree a few years ago with the OU, and as far as assignments and deployments were concerned they were very helpful. I sat one end of year exam whilst in Saudi on detachment, the OU were very understanding and even managed to send a second set of exam papers out in very short notice when the first got lost. I think I ended up taking the exam 24 hours later than the UK, but no problems.
As for credit for military courses it depends on what the courses were. If they gave you a recognised civillian qualification then they should count them, but it's best to approach the OU direct and put the question to them.
Ogre