product.variants
is a collection, so you'll need to loop through it to determine if one of the variants has a title containing 'OS'.
Something like this:
{% assign contains_os = false %}
{% for variant in product.variants %}
{% if variant.title contains 'OS' %}
{% assign contains_os = true %}
{% endif %}
{% endfor %}
<div class="homepage-sizechart">
<div class="sizes">
{% if contains_os %}
{{ 'onesize-triangle.png' | asset_url | img_tag }}
{% else %}
{{ 'size-triangle.png' | asset_url | img_tag }}
{% endif %}
{% for variant in product.variants %}
<span class="{{ variant.title }}-product {% if variant.inventory_quantity == 0 %}outofstock{% endif %}">{{ variant.title }}</span>
{% endfor %}
</div>
</div>