It was his first try since January against Treviso, and the 26-year-old revealed the wait had started to play on his mind.
"It's great to get the record-equalling try," he told the Glasgow Warriors official website. "I've been waiting a little while for it so it's nice to get it off my back.
"It was a great kick by Peter Horne, a lucky bounce and I just had to run to the line, so I was glad to do it."
Usually a regular in the starting lineup since arriving at Scotstoun in 2009, Van der Merwe had been unwell ahead of the Ospreys game so he was pleased to make an impact as a replacement.
"It's only the second or third time I've come of the bench for Glasgow," he said. "I enjoyed coming on at the end of the game.
"I was a little ill in the week leading up to the game but it was great to get on and score at the death, especially after playing so many games being dry."
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