This is the RHS advice
https://www.rhs.org.uk/Plants/116640/Osmanthus-heterophyllus-Goshiki-(v)/Details

it is pruning group 8
so
1. Early flowering evergreen shrubs (Pruning group 8)

Timing: Prune immediately after flowering.
Examples: Berberis, box (Buxus), Camellia, Ceanothus, Choisya, Daphne, Hypericum, laurel (Prunus laurocerasus and Prunus lusitanica), Mahonia, Pieris, Azalea (Rhododendron), Rhododendron, Viburnum tinus.
Pruning: Winter, spring and early summer-flowering evergreens such as Rhododendron and Camellia are best left unpruned except for removal of unsightly shoots and deadheading, unless some shaping is required. Do this by lightly cutting back shoots after flowering. Deadheading and removal of dead, damaged and diseased growth can be done at the same time.
Exceptions: Certain evergreen shrubs require a harder prune to increase flowering or to maintain their attractive shape. Examples include groundcover such as Mahonia aquifolium and Hypericum calycinum which look best sheared over in spring. Deciduous Viburnum - whether winter, spring or summer flowering - also fall within pruning group 8 and can be lightly pruned after flowering unless berries are required. Those shrubs recommended for hedges, topiary or foliage often require regular trimming and tolerate heavier pruning.

hope this helps.

Good question Sophiemears. Excellent answer Seaburngirl. I find the answer very helpful. My Osmanthus has never flowered but it looks great.

Yes. Very helpful, thank you. And never any flowers on mine either. 😕

I find that RHS advice a bit irritating, I've looked it up before - they include Osmanthus in the spring flowering section, which is correct; except that Osmanthus heterophyllus Goshiki flowers in summer, if it flowers at all, so their advice is for general spring flowering shrubs, not this particular shrub.

If yours doesn't flower (I've never seen one with a flower) I'd cut it back this month and reshape, which should give it plenty of time to recover in time for winter, so any brown stuff in the middle won't show by then. I'd not risk cutting it back really hard, because its such a slow growing plant, and give it a handful or three of Growmore raked in at the base immediately afterwards (or whatever general fertilizer you use).
You must have had that some years - I've never seen one that size before!

Mine has very small insignificant flowers from feb to april but perhaps it isn't this species. I must look it up.

It's been there 8 years Bamboo, but took a mega growth spurt last year for some reason.
Thanks for the advice.

I have renovation pruned these on many occasions over the years, they always respond well, i would do this as a rule towards the end of April or early May and with any renovation pruning I always give the plant a heavy watering.

Seaburngirl - the RHS's own page for this particular shrub shows its flowering time as summer and autumn, which is why I'm irritated by the generalised pruning advice...obviously aimed at spring flowering ones.

Regardless of the flowering time, if it needs renovation pruning then you would do it round about now, yes you forgo the flowers maybe depending on the flowering period, but renovation pruning is carried out with the aim of producing a well balanced shrub for the the next few years, you would not do the renovation pruning later in the year for obvious reasons, for some reason people are frightened to renovate shrubs.