Recently got into Only War and as a long standing GM of DH,DW and RT I am finding it probably the better system so far but now a question has arisen,at what point do you get promoted from enlisted to other ranks,btw no military experience so unsure of ranks, should I just randomly promote a character for doing something well or say when you hit 3k,5k,7k exp?

Also No Surrender looks like it gonna be fun, with my parties luck and the way it reads they could die at least 7 times before page 20 but as they are part of a much larger force was going to run it alongside a 40k game with 2 separate people playing that with consequences happening as the party complete objectives,any thoughts?

The subject of promotion and such are covered in both the core rules of OW an its Gm's kit try tying it with the logistics rating of your PC's.

Also have the promotions based on deeds accomplished in game and medals they have earned from combat well from what I have read anyway.

Also No Surrender looks like it gonna be fun, with my parties luck and the way it reads they could die at least 7 times before page 20 but as they are part of a much larger force was going to run it alongside a 40k game with 2 separate people playing that with consequences happening as the party complete objectives,any thoughts?

Oh, they will. The initial breaching action was...messy. Heavy bolters + virtually no cover make for an interesting fight. The Dominate counter-attack was even worse as they managed to get themselves stationed on the front line piquets.

"There's something coming up the corrid...what...the...hell...is....Run! Run, Run, RUN!!!!" <Whirrrrrrrrrr!>

I'd just play "the battle" narratively rather than resolve it. You don't really want to be going back and forth between the two games. Feel free to have two people to go to as the commanders and ask "what's your plan?" but equally, a lot of good ideas (such as patrols on the outer hull and behind the lines raiding) will be out of the scope of a 40k game without a lot of special rules.