What can I use to substitute for sambal oelek?

I find sriracha a bit too sweet with a flavor that can easily overwhelm other ingredients in a dish. You could always make your own chili garlic sauce which is sambal oelek. Andrea Nguyen has two recipes for chili garlic sauce, one cooked and one uncooked. The cooked one is going to be more like sambal oelek that are sold in the supermarket. Both recipes are pretty easy and don't require any special equipment or ingredients, other than ripe chilis. Her blog is vietworldkitchen.com, you can find the recipe in the index under homemade chili garlic sauce.

You could use crushed chilis, the kind people sprinkle on pizza; or 'hoagie spread' or something similar -- crushed hot cherry peppers in vinegar (similar to Sambal Oelek) made for spreading on sandwiches. Looking at the labels on all three, I see they all use preservatives. Homemade versions are the alternative.
Food52 has a Sriracha recipe that is one of the most viewed recipes on the site. (use search box, top right)

My only problem with sritacha available is it is full of preservatives. If you have no problem with that, go for it. Otherwise I make my own. You can find many recipes on the web.

If your supermarket has an Asian section you can substitute sriracha (aka rooster sauce) or a Chinese garlic hot pepper blend.