Question: In the absence of sunlight, plants give out the greenhouse gas, carbon dioxide. So why is it said that plants increase the concentration of atmospheric oxygen? Do they not increase the concentration of carbon dioxide and use the oxygen produced in photosynthesis.

Answer: 
The tag you used on the question shows you know about photosynthesis, but it seems you don't fully recognize its significance. Yes, plants use oxygen and release CO2 all the time for energy use, but they absorb much more CO2 (and release much more O2) while storing energy during the daytime than that 24-hour energy usage undoes.

While plants release CO2 as well as absorb it, they do much more of the latter. By [one scientific estimate](https://en.wikipedia.org/wiki/Carbon_fixation#Net_vs_gross_CO2_fixation), of all the the CO2 plants remove for photosynthesis, they release 40% of it back into the atmosphere. But that means that 60% of that CO2 absorption is never released back into the atmosphere by the plants. So, overall, the net change is that plants remove CO2 from the atmosphere (unlike animal metabolism and combustion which only add CO2 and remove none). 

All those plant stems, branches, and trunks you can see are built out of CO2 that the plant will never release back into the atmosphere. The only CO2 the plants release is CO2 that they previously absorbed from the atmosphere.
Plants start to grow first, from tiny size (from seed in higher plants), and from there, any size in later stages they achieve to have, they achieve it only by first absorbing all the Carbon they need, from air first.
So huge part of plant's later dried weight is actually made from carbon. And all that carbon was incorporated into it's body through continuous photosynthesis - so it was "sucked" from CO2 in the air around the growing plant.

Importantly:

Unlike us, plant can not normally absorb carbohydrates (sugars, fat etc - we eat them, plants don't) in order to use them for metabolism and respiration (ie breathing). For this they can use only carbohydrates they themselves already created first from CO2, through photosynthesis.

Even when they can not use photosynthesis anymore (because of age or in the dark) - they use for respiration only that carbon which they stored in the body before by catching it from air.

And as long as the plant continues to grow and have access to any light it continues to absorb much more CO2 than it releases through respiration. So in balance it keeps absorbing CO2 and releasing oxygen in net balance.

As an mental exercise: It would be different if You would take, let say same house pot plant and place it into a completely dark room for a long time. That plant there is without light, while it already has body which it created previously through photosynthesis, it now can not capture any light and any carbon any more, yet it would still need to be continually breathing. So as this particular plant in the dark, have no source of energy for photosynthesis, and it still is breathing, so in this particular case it would produce CO2.

Practically something resembling such theoretical example happens with significance only, when we store plant body-parts like fruits and vegetables (or potatoes) for a long time. Fruits, vegetables and potatoes (often stored in the dark) breath and not photosynthesize. 

The cycles between a day and night are also bringing plants for short time into the dark and as such they have negative balance (net producing CO2) for a short while, but it is insignificant from the whole day perspective (overal they consume much more CO2 than they produce it) 

Interestingly, there also exist very rare species of parasitic plants (they are parasites of other plants) which are not having their own photosynthesis - in this case they have no chlorophyll and suck sugars out of other plants - and yes, they only breath and yes, they only produce CO2, so they would be probably the only plants which are net producers of carbon dioxide.