Liverpudlian Matthews - who is Scottish qualified courtesy of his Glaswegian mother - joins the club from Boroughmuir, having caught the eye over the course of the 2018/19 Tennent's Premiership season.
The 26-year-old ended the season as the league's top try scorer, crossing for 19 tries in a season that saw his side finish seventh in the final standings.
The hooker's try-scoring prowess also shone through during a spell at Sedgley Park, where he scored 29 tries in 26 appearances during the 2016/17 season.
He has previously been involved with Scotland at U18 level, as well as turning out for the Scotland Club XV.
Speaking to glasgowwarriors.org, Matthews said: "I'm absolutely delighted to sign.
"Glasgow is one of the biggest clubs in Europe and I'm looking forward to hopefully getting a few games under my belt while I'm here.
"I like to get my hands on the ball, attack and be physical around the park. My goal is just to keep my head down and work hard, and give my all for the club."
O'Toole came through the ranks at Connacht and arrives at Scotstoun following a spell in the USA's Major League Rugby competition with Houston Saber Cats.
The 24-year-old made 12 appearances for the Connacht Eagles in the British and Irish Cup, as well as earning Ireland U19 recognition.
He is also eligible to represent the USA, and was selected for the USA Select XV in the Americas Pacific Challenge.
"It's a great opportunity for me," O'Toole told glasgowwarriors.org.
"I'm really pleased to be here. To be going back to Ireland and seeing a few familiar faces this weekend is a great way to start."
Meanwhile, the club can also confirm that second-rower Lars Morrice has been training with the squad and will be available for selection this weekend.
The 24-year-old New Zealander played 10 times for Ayr in the Tennent's Premiership last season and was this morning confirmed as one of Ayrshire Bulls' new recruits ahead of the inaugural Super6 campaign.
He has previously played for Bay Steamers, Chiefs Development and Hong Kong Scottish.
The squad for tomorrow's pre-season encounter with Ulster will be named later today.
Speaking to the club's website, Glasgow Warriors Assistant Coach, John Dalziel welcomed the new additions to the squad.
"We're delighted that Johnny and Pat have joined us for the next few months," he said.
"Both men are dynamic with ball in hand and have shown up well in pre-season training, so we're looking forward to seeing what they can do in a Glasgow Warriors shirt.
"With Fraser (Brown), George (Turner) and Mesu (Dolokoto) going to the Rugby World Cup, they will provide healthy competition at hooker.
"Johnny is a Boroughmuir player and in partnership with them we've been able to bring him in on a short-term deal. We'd like to thank Boroughmuir for their support in allowing Johnny to take up this opportunity with us.
"Lars has also been working hard over the summer and it'sgreat to have him available for selection this weekend too.
"Lars and Johnny are players we became aware of thanks to our close links with the club game."