From what that pdf linked above says, unless you're in Engineering, you need 4.5 units per term to be eligible for OSAP. If you're taking a six-unit course over two terms, then yes, you'd need to add another course per term to be eligible for that term.
Because OSAP lending seems to be based on your course load per term as opposed to the whole summer, I'd imagine you have the option of just adding one course to one term to be eligible for OSAP for that term.
Alternatively, if you find a three-unit course that stretches itself out over the whole summer, you could take that too since you only need 4.5 units per term to be eligible. I haven't bothered to check for such a course, but I can't imagine one existing.
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