Generally speaking, you'll only get an upgrade if they are chronically overbooked and cannot solve the problem by denying people boarding voluntarily. As I'm sure you know, European airlines tend to be less than liberal with operational upgrades. To give you an example, I've been a Senator (Star Gold) on the LH and OS program for over 2 years and can only recall 2 op-upgrades (and one of these was with United!).

The good news is that, if they are chronically overbooked, then you should be near the top of the queue. DEL is somewhat more likely to have this problem than a lot of other OS routes.

Are you checking in at VIE? If so, then there are a couple of dedicated *Gold / Senator desks which you should aim to use.

My tips would be to very nicely ask the check in agent whether the flight is looking full in economy, and mentioning that if it is then are they running a waitlist for upgrades or denied boarding. Be warned that the slightest sign that you are "expecting" anything due to your status in this regard will get taken the wrong way.

When you get to the lounge (the Senator Lounge is the one on the ground floor, after passport control, in the A Concourse (do not go to the lounge before passport control as this tends to be rather more crowded), again ask how the flight is looking. In my experience, the lounge agents tend to have rather more sway than the check in staff in my view.

Allow plenty of time to get to the gate, as you will clear security only as you enter the gate area. If you had no luck at the lounge then it is unlikely you will be getting the upgrade, but if the answers you have got to date indicate you might have a chance then it is worth asking again here.

I would stress that the key thing is to make it look like the upgrade would be a nice extra, but is not something you are trying too hard for.

Whichever section of the cabin you end up in, enjoy the flight!