To ensure the longevity of your arrangement of 20 O'Hara roses in a glass vase, consider the following flower care tips:

1. **Trimming Stems**: Upon receipt, trim the stems at an angle with sharp scissors to facilitate water absorption.

2. **Clean Vase**: Use a clean vase and fill it with lukewarm water. Remove any foliage that may be submerged to prevent bacterial growth.

3. **Water Level**: Keep the vase filled with enough water to cover at least half of the stems. Check the water level daily and replenish as needed.

4. **Location**: Place the vase in a cool, well-lit area away from direct sunlight and heat sources. Avoid placing them near ripening fruits, which release ethylene gas and can shorten the roses' lifespan.

5. **Changing Water**: Replace the water every 2-3 days to maintain freshness. Before adding fresh water, re-trim the stems to promote water uptake.

6. **Pruning**: Remove any wilted or discolored petals and foliage to maintain the arrangement's appearance.

7. **Enjoyment**: Display your arrangement in a prominent location and enjoy the beauty of the O'Hara roses. With proper care, they should last for several days, bringing joy to your space.

By following these care instructions, you can ensure that your arrangement of 20 O'Hara roses remains fresh and vibrant for as long as possible.