One cause of yawning can certainly be that your body needs more oxygen, so key to solving the problem is getting enough breaths in.
Some scores have breaths marked, but for others you'll need to work out for yourself where you can fit your breaths in.
You can train for bigger breaths but some people find it difficult to breathe properly.
I know a couple of people who were asthmatic and had this problem, and they found swimming helped them learn to breathe more deeply and take deeper, more controlled breaths.
music.stackexchange.com/users/104/Doktor Mayhem
One of the first thing you could do is to train yourself to yawn : yawn and alternatively inhale and intonate at a standard voice level at the same time, opening the mouth as wide as possible.
Do that slowly, several times before starting to sing.
It will flex your face muscles in a good way and place your lower jaw.
A little side benefit is that you will be less likely to yawn after this exercice.
Our singing teacher used to make us do this and a little series of other warmups before every session.
music.stackexchange.com/users/48/ogerard
You could also have an iron deficiency, which affects the amount of oxygen your system can use.
I'd say go and talk to a GP and see if it's possibly medically related as well as a technique thing.
music.stackexchange.com/users/84/Nick Bedford
More generally about breathing properly while singing: Stand up straight.
Don't raise your chest when breathing.
Instead, breathe down into your stomach.
Your lungs will expand downwards and you'll have more control over the release of air as you sing.
If you put one hand on your chest and the other on your stomach, the chest hand should remain still and the stomach hand should move out when you breathe in and in when you breathe out.
This should allow you to get more oxygen out of your breaths, regardless of how many you take.
music.stackexchange.com/users/558/christinealittlebit
I experienced the same thing when I was first learning to sing.
I think that yawning is your body's natural reflex to open up wide, flex and then relax all the muscles in your larynx, throat and mouth (all of which you need to remain flexible and in control to sing properly).
Yawning is good for warming up before singing, but not good if it interrupts your singing.
Back then I found that I would yawn a bit at the beginning of rehearsal, but I never yawned during performance because I focused mentally on giving the best performance that I could, and got so busy focusing on forming each note that yawning was no longer a possibility.
music.stackexchange.com/users/None/
Yawning while singing is not necessarily a sign of improper breathing.
Rather, it could be a sign of good throat position.
Proper vocal technique across a wide variety of styles (including classical, folk, and pop, all of which I'm trained in) has a singer open their throat and raise their palate in the rear, similar to the motion you make while yawning.
This helps keep the sound from being overly nasal.
Of course, yawning while singing is not really a great thing if it affects your singing.
There are several things you can do to help.
Firstly, learn to sing through a yawn.
You'd be surprised how, with a little training, you can sing straight through without losing tone while you're yawning.
Another thing you can do is learn what exactly triggers you to yawn while you sing and modify your tongue and throat position slightly -- you want your mouth and throat to be shaped similarly to a yawn, but it's possible you're going a bit too far and actually triggering a yawn unintentionally.
Proper breath support can also help decrease our propensity to yawn while singing, though, as I mentioned, I'm very reluctant to say that yawning is
caused by poor breathing.
music.stackexchange.com/users/19344/Greg Jackson
Singing almost always make me yawn.  I've heard this is due to not breathing correctly, but I'm not sure what that means.  What's the correct way to breathe while singing so I don't yawn?
Why do I yawn while singing?
music.stackexchange.com
<voice><breathing>
music.stackexchange.com/questions/654
music.stackexchange.com/users/173/BenV
One cause of yawning can certainly be that your body needs more oxygen, so key to solving the problem is getting enough breaths in. You can train for bigger breaths but some people find it difficult to breathe properly. One of the first thing you could do is to train yourself to yawn : yawn and alternatively inhale and intonate at a standard voice level at the same time, opening the mouth as wide as possible.  Another thing you can do is learn what exactly triggers you to yawn while you sing and modify your tongue and throat position slightly -- you want your mouth and throat to be shaped similarly to a yawn, but it's possible you're going a bit too far and actually triggering a yawn unintentionally.  One way to stop yawning whilst singing would be to get more oxygen. This can be achieved by training for bigger and more breaths. To help with this, training to yawn and learning what triggers you to yawn may help with this. 
One way to stop yawning whilst singing would be to get more oxygen. This can be achieved by training for bigger and more breaths. To help with this, training to yawn and learning what triggers you to yawn may help with this. 