Me too I have a hard time to decide how to develop the left but O4 is still too close to White's thickness I think
I find this interesting because where you see thickness I see a potential weak or heavy group.
As objective reasons for calling it potentially weak, it does not have any guaranteed eyes at all yet and is bounded by strong Black groups on both sides. If Black plays a move against it, White probably has to defend (or else accept a truly weak group), but his defensive move would be purely that - there is no move to make territory while defending.
I am also influenced in my opinion by a growing realisation that pros see weak groups much earlier than I do. To some extent that must be because they have far more terrifying weapons than I do, and so can go after lions where I can only go after rabbits, but I think also a big part of the reason is that we amateurs tend to think of weak groups as anaemic, weedy clusters that have to live urgently, whereas pros seem to count many groups that allow a powerful forcing move (as is the case here) as weak.
In this particular case, I would also argue that the White group cannot really be regarded as thick because it has no potential to do what thickness normally allows you to do. Black has no weak groups roundabout yet, and White can't even surround any territory with it. There is possible influence in the centre but it is too nebulous and too easy for Black to circumvent.
If you accept that thinking, an invasion at 'd' looks horrendous: it would lead to a splitting attack on two White groups. In that light a shoulder-hit reduction at O4 on the lower side looks more promising. If the reducing group can link up with the White group above it could give him some proper centre thickness. But that seems speculative, and seems to hand the initiative over to Black too readily.
So the right side (and maybe a quiet move there) seems more worthy of attention. This is still fairly open and so can be seen as a virtual territory of three units. Normally the idea is that out of two units of virtual territory, you can expect to get one (the opponent doesn't get much of the other but at least destroys it). With three units it much more problematical to come up with a good plan. The shimari at C15 looks good in that it takes one third of the virtual area at once and reduces what is left to two units. But that means White is looking at not much more than 40 points there and less than 10 in the upper right, with no immediate prospects of more anywhere else (all because of the inefficient White group on the right). Of course he gets the komi as well. Black's lower side can be treated as a three-unit virtual territory where he gets two thirds, so all in all he will at least match White's territory. But has the potential to make some more on the right, and his lower side currently looks easier to grow than White's left side.
On that basis, it seems to me White has to up the stakes, and so a quiet move like C15 or C10 could verge on being a slack move. I too would be looking at F5. It increases the potential value of the left side, overconcentrates Black and heightens the value of a subsequent reducing move. It also extends a helping hand to the right-side White group and has a good follow up around F10. The problem with C15 is that it can too easily turn into gote, so I'd be looking for tesujis to turn this into sente, but even if they don't exist, I think F5 may be enough to keep White in the game. At the very least it's liable to make Black enter the left side at once, and depending on how that goes White might get enough thickness there to exploit Black's potential weakness around M16.
? I suppose the bottom left could be pressured without the direct connection, but I don't see how, exactly.
in your diagram already seems like good timing, as it is an almost pure territory move.