I assume that you work in Seattle and the SST is Seattle Sick Time and not something else. When you call in sick both OTH and SST will be used. If you only have one or the other left you will still be paid for the day. There is a maximum number of hours of Seattle Sick Time that carries over from year to year, I do not remember the maximum. The City of Seattle government website has an faq for the Seattle sick time law http://www.seattle.gov/Documents/Departments/CivilRights/psst-faq-0515.pdf.
Dennis