Hi Katy,
Almost two weeks ago I was going through this - sadly lost my Mum on the 12th. She had colon cancer which had metastatised (sp?) to her lungs and body.
What happened with my Mum is that on the 5th she had a brain haemorrhage and was unresponsive all day, they operated as just the day before she was fit, ready to come home and energetic (she only got diagnosed in Jan). After the op she was fine and practically her old self, but progressively became more sleepy, less co ordinated and more confused throughout the week until she was at the stage your father is at now - her stats were low and she too was on oramorph. Her last day, she was very very sleepy - didn't respond at all and just slept. The doctors decided not to medically intervene. In the night she had several seizures and by morning her breathing was very heavy and her heartbeat very fast. Eventually, it slowed down quite peacefully until her breathing and heart stopped altogether - the last hour was a very smooth transition as crude as it sounds. Almost like she fought and fought and then just relaxed. We had time to hug and say goodbye.
I can't say it will be the same with your Dad, but from what other posters have said and my experience I would say it's likely that there will be a peaceful transition to the end - maybe a little bit distressing for you if his breathing quickens or there are seizures, but the end is smooth and it is a relief to know they were at peace in the last moments. I think you will know when those moments will be, so you can be there with him if you wish or if it is possible.
I hope this provides some comfort in what is truly an unimaginably difficult time. For me, the ending was like a big relaxation for my mum and it made saying goodbye easier to "see" that it was peaceful for her. I don't really know how she died, and I'm not sure I want to because it's more relaxing to feel that her brain, heart and lungs just let go and now she's not stressed or in pain or sick but rather lying on a beach with a white wine spritzer, after fifty seven years of never really getting to relax!
Please PM me if you want to chat - I've handled it pretty well so you won't be upsetting me and it has been only two weeks so I can really understand.
All the best Katy, remember it's okay to not always be strong, to feel vulnerable and to let people help you :)