General advice please

Hi all,
have been reading the post for some time, but this is my first.
I am a 60 year old male, who has lived alone 4 years since divorce.
I was advised to loose some weight because of a knee problem,
I work in a hospital, and asked for advice on which diet to follow,
and was told it would be better to change my eating and life style habits,
rather that use a diet plan and then most likely
put it back on when going back to my old ways.

I have reduced the portion size and gone to lower fat food, no sugar, no
puddings, threw away the frying pan, now grill.
and six months later I have gone from nearly 17 stone at 5'7" to 14 1/4,
and feel much better, cycle most of the time.

in general is olive oil highly fattening?
I have replaced my normal snack type treats with mixed nuts and sultanas,
are there ok within reason?

thanks for any feed back.
Mick.