Creating an if then clause in SQL

I'm fairly new to SQL and I am am used to writing formulas in Crystal Reports. I want to recreate the following formula from Crystal Reports into an SQL statement to create a column:

if (not isnull ({OT.TITLE})) and (not isnull ({OA.LASTNAME}))then {OA.LASTNAME}& ", "& {OT.TITLE} else
if (not isnull ({OT.TITLE})) and (isnull ({OA.LASTNAME}))then {OT.TITLE} else ""

I created the following statement, but it only works if the title and last name fields are both filled out. Otherwise there is an unnecessary ,:

SELECT concat(OA.Lastname, ', ', OT.Title)
FROM OA
LEFT JOIN OT 
    ON OT.ObjectID = OA.ObjectID

Basically I need a column that shows the Last name and title separated by a comma. If there is no Last name, then show the title. If there is no title then show nothing, even if there is a last name. I would really appreciate any help you can give me. I am unfortunately working with SQL 2014.