Older leaves do normally yellow and drop off. Looks like nutrients are low and being transferred to new leaves and new sprouts at the base. If you are just worried about the lack of density. Pruning and moderate application of Nutrients will help. Shrubs in nature drop lower leaves, and try to achieve the best light capture form. Just be skilled at your irrigation habits, This is a drought tolerant Shrub. I think native to South America

Thanks Dale, what ratio of nutrients would you suggest? I have never fertilized it. Also, do you have suggestions on pruning it? I know nothing about pruning, other than cutting away dead growth.

Usually people only see the outside of Oleanders because they are sheared as hedges all the time. Its a good idea to look at shrubs in the forest to see their natural form.

Thanks Dale, I'll read up on pruning at that link and elsewhere!

one more link caring for Oleanders. http://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm

Remember the sap is toxic. Once you pruned it back. Pinching the new growth frequently will keep it fuller in the future.

Ajax I grow many oleanders, You must repot that plant, if you want it to get bigger and be healthy, bump it up to a pot 2 times the size you have it in with a good draining potting soil, no fancy miracle grow just plain potting soil . And then when you prune do it just above where a leaf is , this promotes new growth repeat and next year you'll have a full flowering oleander. Dont over water, but keep it a little moist, spray the leaves with plain water in hot summer days to reduce stress especially on windy days. These plants can take hard cut back pruning, just not after Sept. and you can prune them also in spring to summer. Your plant will be fine, but make it happy give it a new bigger pot with fresh soil.Your plants is root bound and needs to grow. You wont be sorry.Everything Dale said is right, leaves will yellow when they get older and fall off, but being you have not repotted this plant in two years or ever it is time to do it, rule of thumb less fertilizer is better than to much.. Happy planting .... From, Texas Gulf coast.

I have learned not to trust the original pot mix too long. It is designed to be cheap to get the plant to the store and sold off. One way I observed this is that even with just a few days or weeks at the store (depending on how long it was at the store), once I get it home and water it, the water runs right out. Not so great if the weather a plant is growing in will be hot, dry weather.

However, I understand that not everyone wants to change the potting mix right when buying a new plant, so at the minimum, I would have repotted long-living plants like the oleander in the next growing season.