There seems to be a growth on my dad's olive tree - any ideas

I would say this is scale aphids.
Google it to see how to deal with it.

I would have guessed scale insect too Doctorbob but they look a bit too raised for that dont you think?

There are lots of types of scale aphids, as they are now in the greenhouse on the olive tree, they may have develped more than normal. I've seen them recently on a rambling rose looking at least that size.

I have had these on my Olives. best flicked off with the finger nail or substitute at this time of year you may remove them permanently,

I had a simular thing on my willow trees about 2 years ago, as Dimapps describes they are raised bumps on both side of the leaves which suggests it is in the centre of the leaf itself not on the surface and when opened nothing inside. mine were red in colour about half the size of a ladybird! they intigued me! you could not flick them off as Poaannua suggests as they were part of the leaf not a bug or scale insect as Milky suggests in looking up my books I came to the conclusion that they were leaf Galls which occur on all types of trees or shrubs and it is a reaction to the irritation caused by pests or fungi. In some cases they are large and colourful even decorative which mine were. It said in nearly all cases there is no harm to the plant cut off and burn if unsightly, so I just picked off the infected leaves and gave the tree a spray with a bug spray and I have not had it since. can some one tell me is this the same as this Olive tree is suffering or is it a different complaint?

Without actually seeing the little pests it is very difficult to be sure,
As Telme says hers was pest damage and spraying uoutside killed them preventing further damage.
If they don't flick off as Poannua suggests, wash all the leaves with a fungicide, with gloved hands and a face mask. Ventilate well.
Hope we can solve this for you.

I too would go for black scale Drbob if it weren't for their apparent predilection with the leaves which is not usual. Black scale, which is fond of olive trees, usually sticks to the stem. Di. can you look carefully at them and see if any of the bumps has what looks like the letter 'H' embossed on them?

John.