I'm not sure I'll be much help really, as we trained Harry to go outside from day one....
Of course you know by now when little Oscar's triggers are. When we were training Harry, when he did wee-wee's or poo-poo's on the grass, we would say "good wee-wee" etc., hold a small party, give him a treat, and then take him in
He got the hang of it
really quickly (by that, I mean he was completely house-trained in about a fortnight). Being at home really,
really helps of course.
Even now, if we're somewhere that he's never been before and he does his business, I still say "good wee wee, Harry" - people look at me as if I'm completely nuts
but I guess it's just a habit I've gotten into....
Good luck! Sally + Harry x